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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2016	Feb. 28, 2017	Jun. 30, 2016
Document and Entity Information
Entity Registrant Name	Clean Energy Fuels Corp.
Entity Central Index Key	0001368265
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		149,591,164
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 338,632,713

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 36,119	$ 43,724
Restricted cash	6,996	4,240
Short-term investments	73,718	102,944
Accounts receivable, net of allowance for doubtful accounts of $1,895 and $1,063 as of December 31, 2015 and 2016, respectively	79,432	73,645
Other receivables	21,934	60,667
Inventory	29,544	29,289
Prepaid expenses and other current assets	14,021	14,657
Total current assets	261,764	329,166
Land, property and equipment, net	483,923	516,324
Notes receivable and other long-term assets, net	16,377	14,732
Investments in other entities	3,475	5,695
Goodwill	93,018	91,967
Intangible assets, net	38,700	42,644
Total assets	897,257	1,000,528
Current liabilities:
Current portion of debt and capital lease obligations	5,943	149,856
Accounts payable	23,637	26,906
Accrued liabilities	52,601	59,082
Deferred revenue	7,041	10,549
Total current liabilities	89,222	246,393
Long-term portion of debt and capital lease obligations	241,433	352,294
Long-term debt, related party	65,000	65,000
Other long-term liabilities	7,915	7,896
Total liabilities	403,570	671,583
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.0001 par value. Authorized 1,000,000 shares; issued and outstanding no shares	0	0
Common stock, $0.0001 par value. Authorized 224,000,000 shares; issued and outstanding 92,382,717 shares and 145,538,063 shares as of December 31, 2015 and 2016, respectively	15	9
Additional paid-in capital	1,090,361	915,199
Accumulated deficit	(603,836)	(591,683)
Accumulated other comprehensive loss	(17,675)	(20,973)
Total Clean Energy Fuels Corp. stockholders' equity	468,865	302,552
Noncontrolling interest in subsidiary	24,822	26,393
Total stockholders' equity	493,687	328,945
Total liabilities and stockholders' equity	$ 897,257	$ 1,000,528

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,063	$ 1,895
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	1,000,000	1,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	224,000,000	224,000,000
Common stock, issued (in shares)	145,538,063	92,382,717
Common stock, outstanding (in shares)	145,538,063	92,382,717

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Product revenue	$ 351,038	$ 329,168	$ 380,199
Service revenue	51,618	55,152	48,741
Total revenue	402,656	384,320	428,940
Cost of sales (exclusive of depreciation and amortization shown separately below):
Product cost of sales	229,958	230,621	291,462
Service cost of sales	25,592	27,864	17,325
Gain from change in fair value of derivative warrants	(22)	(1,414)	(5,748)
Selling, general and administrative	105,503	113,653	126,435
Depreciation and amortization	59,262	55,219	49,058
Impairment of long-lived asset	0	0	4,772
Total operating expenses	420,293	425,943	483,304
Operating loss	(17,637)	(41,623)	(54,364)
Interest expense	(29,595)	(95,813)	(44,720)
Interest income	827
Other income (expense), net	(306)	2,627	(2,571)
Loss from equity method investments	(22)	(815)	(490)
Gain from extinguishment of debt, net	34,348	0	0
Gain from sale of subsidiary	0	937	11,998
Loss before income taxes	(12,385)	(133,844)	(89,784)
Income tax expense	(1,339)	(1,614)	(1,075)
Net loss	(13,724)	(135,458)	(90,859)
Loss attributable to noncontrolling interest	1,571	1,216	1,200
Net loss attributable to Clean Energy Fuels Corp.	$ (12,153)	$ (134,242)	$ (89,659)
Loss per share:
Basic and diluted (in dollars per share)	$ (0.10)	$ (1.47)	$ (0.96)
Weighted average common shares outstanding:
Basic and diluted (in shares)	119,395,423	91,607,578	93,678,432

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net loss	$ (13,724)	$ (135,458)	$ (90,859)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2014, 2015 and 2016	1,567	(9,653)	(7,958)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2014, 2015 and 2016	1,652	(8,078)	4,866
Unrealized gains on available-for-sale securities, net of $0 tax in 2014, 2015 and 2016	79	6	544
Total other comprehensive income (loss)	3,298	(17,725)	(2,548)
Comprehensive loss	(10,426)	(153,183)	(93,407)
Clean Energy Fuels Corp
Net loss	(12,153)	(134,242)	(89,659)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2014, 2015 and 2016	1,567	(9,653)	(7,958)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2014, 2015 and 2016	1,652	(8,078)	4,866
Unrealized gains on available-for-sale securities, net of $0 tax in 2014, 2015 and 2016	79	6	544
Total other comprehensive income (loss)	3,298	(17,725)	(2,548)
Comprehensive loss	(8,855)	(151,967)	(92,207)
Noncontrolling Interest in Subsidiary
Net loss	(1,571)	(1,216)	(1,200)
Other comprehensive income (loss), net of tax:
Comprehensive loss	$ (1,571)	$ (1,216)	$ (1,200)

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (PARENTHETICAL) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Foreign currency translation adjustment, tax	$ 0	$ 0	$ 0
Foreign currency adjustments on intra-entity long-term investments, tax	0	0	0
Unrealized gains (losses) on available-for sale securities, tax	0	0	0
Unrecognized gains on derivatives, tax	$ 0	$ 0	$ 0

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Subsidiary
Beginning balance (in shares) at Dec. 31, 2013		89,364,397
Beginning balance at Dec. 31, 2013	$ 518,538	$ 9	$ 883,045	$ (367,782)	$ (700)	$ 3,966
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of offering costs (in shares)		838,947
Issuance of common stock, net of offering costs	6,050		6,050
Exercise of additional membership interest in subsidiary	2,363		2,363
Stock-based compensation	11,514		11,514
Foreign currency adjustments on intra-entity long-term investments converted to equity	(4,866)		(4,866)
Acquisition of non-controlling interest in subsidiary	28,075					28,075
Sale of non-controlling interest in subsidiary	(3,232)					(3,232)
Net loss	(90,859)			(89,659)		(1,200)
Accumulated other comprehensive income (loss)	(2,548)				(2,548)
Ending balance (in shares) at Dec. 31, 2014		90,203,344
Ending balance at Dec. 31, 2014	465,035	$ 9	898,106	(457,441)	(3,248)	27,609
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	10,779		10,779
Net loss	(135,458)			(134,242)		(1,216)
Issuance of common stock (in shares)		2,179,373
Issuance of common stock	6,314	$ 0	6,314
Accumulated other comprehensive income (loss)	$ (17,725)				(17,725)
Ending balance (in shares) at Dec. 31, 2015	92,382,717	92,382,717
Ending balance at Dec. 31, 2015	$ 328,945	$ 9	915,199	(591,683)	(20,973)	26,393
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of offering costs (in shares)		32,889,517
Issuance of common stock, net of offering costs	101,120	$ 4	101,116
Stock-based compensation	8,092		8,092
Net loss	(13,724)			(12,153)		(1,571)
Issuance of common stock in connection with debt extinguishment (in shares)		20,265,829
Issuance of common stock in connection with debt extinguishment	65,956	$ 2	65,954
Accumulated other comprehensive income (loss)	$ 3,298				3,298
Ending balance (in shares) at Dec. 31, 2016	145,538,063	145,538,063
Ending balance at Dec. 31, 2016	$ 493,687	$ 15	$ 1,090,361	$ (603,836)	$ (17,675)	$ 24,822

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (13,724)	$ (135,458)	$ (90,859)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	59,262	55,219	49,058
Provision for doubtful accounts, notes and inventory	4,374	2,656	1,277
Derivative gain	(22)	(1,414)	(5,748)
Stock-based compensation expense	8,092	10,779	11,514
Amortization of debt issuance cost	1,527	2,969	4,194
Non-cash interest charge related to a terminated credit agreement	0	54,925	0
Accretion of notes payable	0	57	412
Gain on extinguishment of debt, net	(34,348)	0	0
Long-lived intangible impairment	0	0	4,772
Gain on sale of subsidiary	0	(937)	(11,998)
Gain on contingent consideration for acquisitions	0	0	(208)
Changes in operating assets and liabilities, net of assets and liabilities acquired and disposed:
Accounts and other receivables	30,171	3,426	(55,573)
Inventory	(1,520)	5,407	(979)
Prepaid expenses and other assets	347	2,876	1,361
Accounts payable	(764)	(12,005)	9,126
Accrued expenses and other	(7,333)	(596)	7,646
Net cash provided by (used in) operating activities	46,062	(12,096)	(76,005)
Cash flows from investing activities:
Purchases of short-term investments	(137,023)	(158,840)	(157,629)
Maturities and sales of short-term investments	165,695	176,969	171,902
Purchases of and deposits on property and equipment	(23,640)	(51,415)	(88,628)
Loans made to customers	(2,816)	(4,279)	(9,140)
Payments on and proceeds from sales of loans receivable	842	928	6,580
Restricted cash	(2,634)	1,650	(3,567)
Cash received with sale of subsidiary, net of cash transferred	0	1,118	39,760
Investments in other entities	(833)	0	(6,634)
Capital from equity method investment	3,031	0	0
Acquisitions, net of cash acquired	(1,550)	0	467
Net cash provided by (used in) investing activities	1,072	(33,869)	(46,889)
Cash flows from financing activities:
Issuances of common stock	103,591	7,197	2,300
Fees paid for issuances of common stock	(2,283)	(883)	0
Proceeds from debt instruments	7,412	384	12,778
Proceeds from revolving line of credit	73,508	31	34,607
Proceeds from exercise of additional membership interest in subsidiary	0	0	6,992
Repayment of borrowings under revolving line of credit	(50,027)	(64)	(40,354)
Repayment of capital lease obligations and debt instruments	(187,824)	(6,258)	(41,036)
Contingent consideration paid relating to business acquisitions	0	0	(176)
Payment for debt issuance costs	0	0	(896)
Net cash provided by (used in) financing activities	(55,623)	407	(25,785)
Effect of exchange rates on cash and cash equivalents	884	(3,099)	1,027
Net decrease in cash and cash equivalents	(7,605)	(48,657)	(147,652)
Cash and cash equivalents, beginning of year	43,724	92,381	240,033
Cash and cash equivalents, end of year	36,119	43,724	92,381
Supplemental disclosure of cash flow information:
Income taxes paid	1,012	890	943
Interest paid, net of $3,160, $835 and $447 capitalized, respectively	$ 29,774	$ 37,662	$ 39,224

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Cash Flows [Abstract]
Interest paid, capitalized	$ 447	$ 835	$ 3,160

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2016

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Summary of Significant Accounting Policies

The Company

Clean Energy Fuels Corp., together with its majority and wholly owned subsidiaries (hereinafter collectively referred to as the "Company," unless the context or the use of the term indicates or requires otherwise) is engaged in the business of selling natural gas as an alternative fuel for vehicle fleets and related natural gas fueling solutions to its customers, primarily in the United States and Canada.

The Company's principal business is supplying compressed natural gas (“CNG”), liquefied natural gas (“LNG”) and renewable natural gas ("RNG") (which can be delivered in the form of CNG or LNG) for light, medium and heavy-duty vehicles and providing operation and maintenance ("O&M") services for natural gas fueling stations. As a comprehensive solution provider, the Company also designs, builds, operates, and maintains fueling stations; manufactures, sells and services non-lubricated natural gas fueling compressors and other equipment used in CNG stations and LNG stations; offers assessment, design and modification solutions to provide operators with code-compliant service and maintenance facilities for natural gas vehicle fleets; transports and sells CNG and LNG to industrial and institutional energy users who do not have direct access to natural gas pipelines; processes and sells RNG; sells tradable credits it generates by selling natural gas and RNG as a vehicle fuel, including credits under the California and the Oregon Low Carbon Fuel Standards (collectively, "LCFS Credits") and Renewable Identification Numbers ("RIN Credits" or "RINs") under the federal Renewable Fuel Standard Phase 2; helps its customers acquire and finance natural gas vehicles; and obtains federal, state and local credits, grants and incentives.

In addition, through December 29, 2014, the Company processed, extracted, and sold RNG from its former McCommas Bluff landfill in Dallas, Texas. See Note 2 for further information.

Basis of Presentation

The accompanying consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments necessary to state fairly the Company's financial position, results of operations, comprehensive loss and cash flows in accordance with accounting principles generally accepted in the United States of America ("US GAAP"). All intercompany accounts and transactions have been eliminated.

Reclassifications

Prior period Interest income of $363 and $843 for the years ended December 31, 2014 and 2015, respectively, were reclassified from Interest expense, net as a separate line item to conform to the classifications used to prepare the consolidated financial statements for the year ended December 31, 2016. This reclassification had no material impact on the Company’s financial position, results of operations or cash flows as previously reported.

During the year ended December 31, 2016, the Company adopted Accounting Standards Update ("ASU") No. 2015-03, Interest - Imputation of Interest, which requires that debt issuance costs be presented in the balance sheet as a deduction from the carrying amount of the related liability, rather than as a deferred charge. The standard is required to be applied on a retrospective basis. As a result of applying the standard, unamortized debt issuance costs of $273 were reclassified from Prepaid expenses and other current assets to Current portion of debt and capital lease obligations and $4,991 were reclassified from Notes receivable and other long-term assets to Long-term portion of debt and capital lease obligations as of December 31, 2015.

Use of Estimates

The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and may result in material effects on the Company's operating results and financial position. Significant estimates made in preparing the consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, fair value measurements and stock-based compensation expense.

Cash and Cash Equivalents

The Company considers all highly liquid investments with maturities of three months or less on the date of acquisition to be cash equivalents.

Fair Value of Financial Instruments

The carrying values of the Company's financial instruments, including cash and cash equivalents, restricted cash, short-term investments, accounts and other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable, approximate their respective fair values.

Inventory

Inventory consists of raw materials and spare parts, work in process and finished goods and is stated at the lower of cost (first-in, first-out) or market. The Company writes down the carrying value of its inventory to net realizable value for estimated obsolescence or unmarketable inventory in an amount equal to the difference between the cost of inventory and its estimated realizable value based upon assumptions about future demand and market conditions, among other factors.

Inventories consisted of the following as of December 31, 2015 and 2016:



	2015		2016
Raw materials and spare parts	$	25,113	$	24,843
Work in process	973		845
Finished goods	3,203		3,856
Total inventory	$	29,289	$	29,544

Property and Equipment

Property and equipment are recorded at cost. Depreciation and amortization are recognized over the estimated useful lives of the assets using the straight-line method. The estimated useful lives of depreciable assets are three to twenty years for LNG liquefaction plant assets, up to10 years for station equipment and LNG trailers, and three to seven years for all other depreciable assets. Leasehold improvements are amortized over the shorter of their estimated useful lives or related lease terms. Periodically, the Company receives grant funding to assist in the financing of natural gas fueling station construction. The Company records the grant proceeds as a reduction of the cost of the respective asset. Total grant proceeds received were approximately $959, $4,292, and $3,295 for the years ended December 31, 2014, 2015 and 2016, respectively.

Long-Lived Assets

The Company reviews long-lived assets, which include property and equipment and intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. Recoverability of long-lived assets to be held and used is measured by comparing the carrying amount of an asset to future undiscounted cash flows expected to be generated by the asset or asset group. Estimated future cash flows are determined by management based on a number of estimates, including future cash flow projections, discount rates and terminal values. In determining these estimates, management considers internally generated information and information obtained from discussions with market participants. The determination of fair value requires significant judgment by both management and outside experts engaged to assist in this process. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the fair value of the assets. In the fourth quarter of 2014, the Company determined that a long-lived asset related to a contract acquired in its acquisition of the business of Clean Energy Compression was impaired and recorded an impairment charge of $4,772. The Company had no impairments of the Company’s long-lived assets during the years ended December 31, 2015 or 2016.

Intangible assets with finite useful lives are amortized over their respective estimated useful lives using the straight-line method. The estimated useful lives of intangible assets with finite useful lives are two to 20 years for technology, one to eight years for customer relationships, one to 10 years for acquired contracts, two to 10 years for trademarks and trade names, and three years for non-compete agreements. In 2014, the Company acquired a controlling interest in NG Advantage, LLC (“NG Advantage”) and allocated approximately $5,600 of the purchase price to the identifiable intangible assets related to customer relationships and trade names.

The Company's intangible assets as of December 31, 2015 and 2016 were as follows:



	2015			2016
Technology	$	54,400		$	54,400
Customer relationships	16,576			16,576
Acquired contracts	3,694			4,384
Trademark and trade names	8,200			8,200
Non-compete agreements	2,060			2,060
Total intangible assets	84,930			85,620
Less accumulated amortization	(30,442		)	(37,591		)
Foreign currency rate change	(11,844		)	(9,329		)
Net intangible assets	$	42,644		$	38,700

Amortization expense for intangible assets was $7,024, $5,539, and $5,794 for the years ended December 31, 2014, 2015 and 2016, respectively. Estimated amortization expense for the five years and thereafter succeeding the year ended December 31, 2016 is approximately $6,956, $5,913, $4,532, $3,835, $3,184 and $14,280, respectively.

Goodwill

Goodwill represents the excess of costs incurred over the fair value of the net assets of acquired businesses. The Company assesses its goodwill using a qualitative approach to determine whether it is more likely than not that the fair value of its reporting unit is less than its carrying value. The qualitative assessment includes the potential impact on a reporting unit's fair value of certain events and circumstances, including the Company's market capitalization value, macroeconomic conditions, industry and market considerations, cost factors, and other relevant entity-specific events. If it is determined, based upon the qualitative assessment, that it is more likely than not that the reporting unit's fair value is less than its carrying amount, then a two-step quantitative impairment test is performed. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review.

The Company is required to use judgment when applying the goodwill impairment test, including, among other considerations, the identification of reporting units and the assessment of qualitative factors involved in the test. Applying this judgment, the Company has determined that it is a single reporting unit and, based upon its qualitative assessment of goodwill, has concluded it is more likely than not that the fair value of its reporting unit exceeds its carrying amount and no further quantitative analysis was warranted. During the years ended December 31, 2014, 2015 and 2016, there were no indicators of impairment to goodwill.

The Company reduced its goodwill balance by $7,205 when it sold its subsidiary Dallas Clean Energy McCommas Bluff, LLC ("DCEMB") on December 29, 2014 and added $21,070 to its goodwill balance when it acquired NG Advantage on October 14, 2014 (all as described in Note 2). The goodwill balances on the consolidated balance sheets include foreign currency translation gains (losses) of $(6,578) and $1,051 as of December 31, 2015 and 2016, respectively. See Note 2 for further information.

Revenue Recognition

The Company recognizes revenue on various products and services. The table below and the following discussion describe the Company’s revenue by group of similar products.



	Year Ended December 31,
(in thousands)	2014		2015		2016
Volume Related	$	247,899	$	260,629	$	283,814
Compressor Sales	84,775		54,497		27,262
Station Construction Sales	67,392		37,830		64,942
VETC	28,359		30,986		26,638
Other	515		378		—
	$	428,940	$	384,320	$	402,656

Volume Related

The Company’s volume related revenue primarily consists of CNG, LNG and RNG fuel sales, RINs and LCFS Credits sales and O&M services. This revenue is recognized when the following four criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred and title and the risks and rewards of ownership have been transferred to the customer or services have been rendered; (iii) the price is fixed or determinable; and (iv) collectability is reasonably assured. Applying these factors, the Company typically recognizes revenue from the sale of natural gas fuel at the time the fuel is dispensed or, in the case of LNG sales agreements, delivered to the customers' storage facilities. The Company recognizes revenue from O&M service agreements as the related services are provided.

The Company generates LCFS Credits when it sells RNG and conventional natural gas for use as a vehicle fuel in California and Oregon and it generates RIN Credits when it sells RNG as a vehicle fuel in the United States. The Company can sell these credits to third parties who need the credits to comply with federal and state requirements. RIN and LCFS Credits are included in volume related revenues. The Company recognizes revenue from the generation of these credits when it has an agreement in place to sell the credits at a fixed or determinable price.

Compressor Sales

The Company recognizes compression revenues through its subsidiary Clean Energy Compression when it sells non-lubricated natural gas fueling compressors and other equipment. Clean Energy Compression uses the percentage-of-completion method of accounting to recognize revenue because its projects are small and it has been able to demonstrate that it can reasonably estimate costs to complete. In these circumstances, revenue is recognized based on costs incurred in relation to total estimated costs to be incurred for a project.

Station Construction Sales

Beginning January 1, 2016, the Company began using the percentage of completion method to recognize revenue for station construction projects using the cost-to-cost method. Under this method, the Company estimates the percentage of completion of a project based on the costs incurred to date for the associated contract in comparison to the estimated total costs for such contract at completion. Historically, the Company recognized revenue on station construction projects using the completed contract method because the Company did not have a reliable means to make estimates of the percentage of the contract completed. Under the completed contract method, the construction projects were considered substantially complete at the earlier of customer acceptance of the fueling station or the time when fuel dispensing activities at the station began. The sale of compressors and related equipment continues to be recognized under the percentage of completion method as in previous periods. Effective January 1, 2016, the Company implemented a cost tracking system that provides for a detailed tracking of costs incurred on its station construction projects on a project by project basis. The Company also changed related accounting activities and processes to timely identify and monitor costs. As a result of this implementation, the Company is able to make reliable estimates as to the percentage of a project that is complete at the end of each reporting period.

Station construction contracts are generally short-term, except for certain larger and more complex stations, which can take up to 24 months to complete. Management evaluates the performance of contracts on an individual contract basis. Contract price and cost estimates are reviewed periodically as work progresses and adjustments proportionate to the percentage of completion are reflected in contract revenues in the reporting period when such estimates are revised.

The nature of accounting for contracts is such that refinements of estimates to account for changing conditions and new developments are continuous and characteristic of the process. Many factors that can affect contract profitability may change during the performance period of a contract, including differing site conditions, the availability of skilled contract labor, the performance of major suppliers and subcontractors, and unexpected changes in material costs. Changes to these factors may result in revisions to costs and income, which are recognized in the period in which the revisions become known. Provisions for estimated losses on uncompleted contracts are made in the period in which such losses become known. During the year ended December 31, 2016, there were no significant losses on open contracts.

The Company considers unapproved change orders to be contract variations for which the customer has approved the change of scope but an agreement has not been reached as to an associated price change. Change orders that are unapproved as to both price and scope are evaluated as claims. Claims have historically been insignificant. There were no significant unapproved change orders, claims, contract penalties, settlements or changes in contract estimates during the year ended December 31, 2016.

As a result of using the percentage of completion method to recognize revenues, revenue and operating income from station construction sales during the year ended December 31, 2016 were higher by $18,683 and $1,666, respectively, than would have been recognized during the period under the completed contract method. Additionally, net loss per diluted share was $0.01 lower than what would have been reported using the completed contract method.

In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction of and sale of a station, providing O&M services to the station, and sale of fuel to the customer. The Company evaluates the separability of revenues based on Financial Accounting Standards Board ("FASB") authoritative guidance, which provides a framework for establishing whether or not a particular arrangement with a customer has one or more revenue elements, and allows the Company to use a combination of internal and external objective and reliable evidence to develop management's best estimate of the fair value of the contract elements. If the arrangement contains a lease, the Company uses the existing evidence of fair value to separate the lease from the other elements in the arrangement. The arrangement's consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the estimated relative selling price of each deliverable, which is determined based on the historical data derived from the Company's stand-alone projects. The revenue allocated to the construction of the station is recognized using the percentage of completion method. The revenue allocated to the O&M services is recognized ratably over the term of the arrangement and sale of fuel is recognized as the fuel is delivered.

See the discussion under “Alternative Fuels Excise Tax Credit” below for further information.

Other

The Company collects and remits taxes assessed by various governmental authorities that are imposed on and concurrent with revenue-producing transactions between the Company and our customers. These taxes may include, but are not limited to, fuel, sales and value-added taxes. The Company reports the collection of these taxes on a net basis.

Alternative Fuels Excise Tax Credit

Under separate pieces of U.S. federal legislation from October 1, 2006 through December 31, 2014, the Company was eligible to receive a federal alternative fuels tax credit (“VETC”) of $0.50 per gasoline gallon equivalent of CNG and $0.50 per liquid gallon of LNG that it sold as vehicle fuel. In December 2015, another alternative fuels tax credit, which the Company also refers to as VETC, was made available for the period from January 1, 2015 through December 31, 2016. This new credit was the same as the credit it replaced, except that the alternative fuels tax credit for LNG sold as a vehicle fuel in 2016 was based on the diesel gallon equivalent of LNG sold rather than the liquid gallon of LNG sold.

Based on the service relationship with its customers, either the Company or its customers claims the credit. The Company records its VETC credits, if any, as revenue in its consolidated statements of operations as the credits are fully payable and do not need to offset income tax liabilities to be received. As such, the credits are not deemed income tax credits under the accounting guidance applicable to income taxes.

VETC revenue for years ended December 31, 2014, 2015 and 2016 was $28,359, $30,986 and $26,638, respectively.

LNG Transportation Costs

The Company records the costs incurred to transport LNG to its customers in the line item product cost of sales in the accompanying consolidated statements of operations.

Advertising Costs

Advertising costs are expensed as incurred. Advertising costs amounted to $439, $44 and $15 for the years ended December 31, 2014, 2015 and 2016, respectively.

Stock-Based Compensation

The Company recognizes compensation expense for all stock‑based payment arrangements, net of an estimated forfeiture rate, over the requisite service period of the award. For stock options, the Company determines the grant date fair value using the Black‑Scholes option pricing model, which requires the input of certain assumptions, including the expected life of the stock‑based payment awards, stock price volatility and risk‑free interest rates. For restricted stock units, the Company determines the grant date fair value based on the closing market price of its common stock on the date of grant.

Income Taxes

Income taxes are computed using the asset and liability method. Under this method, deferred income taxes are recognized by applying enacted statutory tax rates applicable to future years to differences between the tax bases and financial carrying amounts of existing assets and liabilities. The impact on deferred taxes of changes in tax rates and laws, if any, are applied to the years during which temporary differences are expected to be settled and are reflected in the consolidated financial statements in the period of enactment. Valuation allowances are established when management determines it is more likely than not that deferred tax assets will not be realized. When evaluating the need for a valuation analysis, we use estimates involving a high degree of judgment including projected future US GAAP income and the amounts and estimated timing of the reversal of any deferred tax assets and liabilities.

The Company has a recognition threshold and a measurement attribute for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities based on the technical merits of the position. The amount recognized is measured as the largest amount of benefit that has a greater than 50 percent likelihood of being realized upon ultimate settlement. The Company recognizes potential accrued interest and penalties related to unrecognized tax benefit in income tax expense.

The Company operates within multiple domestic and foreign taxing jurisdictions and is subject to audit in these jurisdictions. These audits can involve complex issues, which may require an extended period of time to resolve. Although the Company believes that adequate consideration has been given to these issues, it is possible that the ultimate resolution of these issues could be significantly different than originally estimated.

Net Loss Per Share

Basic net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period. Diluted net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period and potentially dilutive securities outstanding during the period, and therefore reflects the dilution from common shares that may be issued upon exercise or conversion of these potentially dilutive securities, such as stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock awards and warrants is computed under the treasury stock method. The dilutive effect of convertible notes and restricted stock units is computed under the if-converted method. Potentially dilutive securities are excluded from the computations of diluted net loss per share if their effect would be antidilutive.

The following potentially dilutive securities have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive:



	2014	2015	2016
Stock options	11,486,301	11,487,938	11,467,796
Warrants	6,130,682	2,130,682	—
Convertibles notes	35,185,979	35,185,979	16,573,799
Restricted stock units	2,591,752	3,419,776	2,072,304

In 2013, 5,000,000 shares of common stock subject to the GE Warrant, as defined and described in Notes 10 and 12, were included in the basic net loss per share calculation. On September 11, 2014, the Company determined it no longer met certain conditions required to include 4,000,000 of the shares of common stock subject to the GE Warrant in its weighted average share calculations. As a result, from September 11, 2014 to December 31, 2015, the Company (i) excluded 4,000,000 shares of common stock issuable upon exercise of the GE Warrant from the weighted average number of shares outstanding in the basic net loss per share calculations, and (ii) included the remaining 1,000,000 shares of common stock issuable upon exercise of the GE Warrant in the weighted-average number of shares outstanding in the basic and diluted net loss per share calculations, as 500,000 shares became exercisable in 2012 upon the execution of the associated Credit Agreement, as defined and described in Note 10 and an additional 500,000 shares became exercisable on December 31, 2014 in connection with an amendment to the Credit Agreement executed on December 29, 2014. On December 31, 2015, the Company terminated the Credit Agreement. As a result of this termination and as of it effective date, 4,000,000 shares subject to the GE Warrant that were not then vested became incapable of vesting because the vesting conditions relating to these shares could not occur following such termination and, accordingly, the GE Warrant effectively became exercisable for the 1,000,000 shares that were then vested. On October 4, 2016, the holders of the GE Warrant exercised the warrant to purchase the 1,000,000 shares of common stock that were vested and exercisable thereunder pursuant to the cashless exercise provisions thereof, which resulted in the Company's issuance of 997,740 shares of common stock to such holders. Following such exercise, the GE Warrant has been surrendered and canceled in full and the Company has no further obligations under such GE Warrant.

Foreign Currency Translation

The Company uses the local currency as the functional currency of its foreign subsidiaries. Accordingly, all assets and liabilities outside the United States are translated into U.S. dollars at the rate of exchange in effect at the balance sheet date. Revenue and expense items are translated at the weighted-average exchange rates prevailing during the period. Foreign currency translation adjustments are recorded as accumulated other comprehensive income (loss) in stockholders' equity.

Foreign currency transactions occur when there is a transaction denominated in other than the respective entity's functional currency. The Company records the changes in the exchange rate for these transactions in the consolidated statements of operations. For the years ended December 31, 2014, 2015 and 2016, foreign exchange transaction gains and (losses) were included in other income (expense) in the accompanying consolidated statements of operations and were $(3,188), $975 and $132, respectively.

Comprehensive Loss

Comprehensive loss is defined as the change in equity (net assets) of a business enterprise during the period from transactions and other events and circumstances from non-owner sources. The difference between net income and comprehensive loss for the years ended December 31, 2014, 2015 and 2016 was primarily comprised of the Company's foreign currency translation adjustments. During 2014, the Company converted a long-term equity intra-entity investment to equity and the related translation adjustments were reclassified to additional paid-in capital.

Concentration of Credit Risk

Credit is extended to all customers based on financial condition, and collateral is generally not required. Concentrations of credit risk with respect to trade receivables are limited because of the large number of customers comprising the Company's customer base and dispersion across many different industries and geographies. Certain international customers, however, have historically been slower to pay on trade receivables. Accordingly, the Company continually monitors collections and payments from its customers and maintains a provision for estimated credit losses based upon its historical experience and any specific customer collection issues that it has identified. In addition, through Export Development Canada, Clean Energy Compression maintains accounts receivable insurance on a substantial portion of its foreign trade receivables, which covers up to 90% of the related outstanding balance. Although credit losses have historically been within the Company's expectations and the provisions established, the Company cannot guarantee that it will continue to experience the same credit loss rates that it has in the past.

The Company places its cash and cash equivalents with high credit quality financial institutions. At times, such investments may be in excess of the Federal Deposit Insurance Corporation ("FDIC"), Canadian Deposit Insurance Corporation ("CDIC"), and other foreign insurance limits. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash deposits. The amounts in excess of FDIC insurance limits were $40,691 and $34,439 as of December 31, 2015 and 2016, respectively.

Recently Adopted Accounting Changes and Recently Issued and Adopted Accounting Standards

In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new ASU, goodwill impairment testing will be performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value. The new standard is effective for annual and interim goodwill impairment tests in fiscal years beginning after December 15, 2019, which for the Company is the first quarter of 2020 and should be applied on a prospective basis. Early adoption is permitted for annual or interim goodwill impairment testing performed after January 1, 2017. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures.

In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The new standard provides guidance to entities to assist with evaluating when a set of transferred assets and activities (collectively, the "set") is a business and provides a criteria to determine when a set is not a business. Under the new guidance, when substantially all of the fair value of gross assets acquired (or disposed of) is concentrated in a single identifiable asset, or group of similar assets, the assets acquired would not represent a business. Also, to be considered a business, an acquisition would have to include an input and a substantive process that together significantly contribute to the ability to produce outputs. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017, which for the Company is the first quarter of 2018 and should be applied on a prospective basis to any transactions occurring within the period of adoption. Early adoption is permitted for interim or annual periods in which the financial statements have not been issued. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures.

In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The new standard requires restricted cash and restricted cash equivalents to be included as components of total cash and cash equivalents as presented on the statement of cash flows. This pronouncement is effective for reporting periods beginning after December 15, 2017, which for the Company is the first quarter of 2018. Early adoption is allowed in an interim or annual reporting period. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures.

In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. Under the new standard, the selling (transferring) entity is required to recognize a current tax expense or benefit upon transfer of the asset. Similarly, the purchasing (receiving) entity is required to recognize a deferred tax asset or liability, as well as the related deferred tax benefit or expense, upon purchase or receipt of the asset. This pronouncement is effective for reporting periods beginning after December 15, 2017, which for the Company is the first quarter of 2018, with early adoption permitted. The Company will early adopt the standard as of January 1, 2017. The impact of adoption will not have a material impact to the Company's financial statements.

In September 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Payments. The new standard provides clarification as to the classification of certain transactions as operating, investing or financing activities. This pronouncement is effective for reporting periods beginning after December 15, 2017, with early adoption permitted. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard amends the impairment model to utilize an expected loss methodology in place of the currently used incurred loss methodology, which will result in the more timely recognition of losses. This pronouncement is effective for reporting periods beginning after December 15, 2019, which for the Company is the first quarter of 2020. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

In March 2016, the FASB issued ASU No. 2016-09, Compensation – Stock Compensation (Topic 718) ): Improvements to Employee Share-Based Payments Accounting. The new standard was issued to simplify the accounting for share-based payment transactions, including income tax consequences, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. The Company will adopt the standard in the first quarter of fiscal 2017, prospectively. In connection with the adoption, the Company will elect to recognize forfeitures when they occur. Previously, the Company estimated a forfeiture rate in accordance with prior guidance. This election will be implemented under the modified retrospective approach, resulting in an immaterial decrease to retained earnings representing the cumulative additional compensation expense that would have been amortized through the date of adoption had this accounting policy election been in place.

In February 2016, the FASB issued ASU No. 2016-02, Leases. The new standard requires most leases to be recognized on the balance sheet which will increase reported assets and liabilities. Lessor accounting remains substantially similar to current guidance. The new standard is effective for annual and interim periods in fiscal years beginning after December 15, 2018, which for the Company is the first quarter of 2019, and mandates a modified retrospective transition method. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The new standard requires equity investments to be measured at fair value with changes in fair value recognized in net income, simplifies the impairment assessment of equity investments without readily determinable fair values, eliminates the requirement to disclose the methods and significant assumptions used to estimate fair value, requires use of the exit price notion when measuring fair value, requires separate presentation in certain financial statements, and requires an evaluation of the need for a valuation allowance on a deferred tax asset related to available-for-sale securities. The new standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

In May 2014, the FASB issued an accounting standard update related to revenue from contracts with customers, which, along with amendments issued in 2015 and 2016, will provide a single, comprehensive revenue recognition model for all contracts with customers. The underlying principle is to recognize revenue when promised goods or services are transferred to customers in amounts that reflect the consideration that is expected to be received for those goods or services. The new standard also requires entities to enhance disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. This accounting standard update, as amended, will be effective for annual reporting periods after December 15, 2017, which for the Company is the first quarter of fiscal 2018. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized in retained earnings as of the date of adoption ("modified retrospective basis"). The Company expects to adopt this accounting standard update on a modified retrospective basis and is currently evaluating the impact of this accounting standard update on its consolidated financial statements and related disclosures.

Acquisitions and Divestitures

12 Months Ended

Dec. 31, 2016

Business Combinations [Abstract]

Acquisitions and Divestitures

NG Advantage

On October 14, 2014, the Company entered into a Common Unit Purchase Agreement ("UPA") with NG Advantage. NG Advantage is engaged in the business of transporting CNG in high-capacity trailers to industrial and institutional energy users, such as hospitals, food processors, manufacturers and paper mills, that do not have direct access to natural gas pipelines. The Company viewed the acquisition as a strategic investment in the expansion of the Company's initiative to deliver natural gas to industrial and institutional energy users. Under the terms of the UPA, the Company paid NG Advantage $37,650 for a 53.3% controlling interest in NG Advantage. $19,000 of the purchase price was paid in cash on October 14, 2014 and the remaining $18,650 of the purchase price was paid in the form of an unsecured promissory note issued by the Company (the "NG Advantage Note"). The principal amount of the NG Advantage Note was paid by the Company in two payments as follows: (i) $3,000 was paid on January 13, 2015 and (ii) the remaining $15,650 was paid on April 1, 2015. The NG Advantage Note did not bear interest. The fair value of the NG Advantage Note delivered to NG Advantage is excluded from the Company's consolidated statements of cash flows as it is a non-cash investing activity. The consideration paid is accounted for as an intercompany transaction, as NG Advantage's financial results are included in the Company's consolidated financial statements.

The Company recognized the assets acquired and the liabilities assumed, measured at their fair values, as of the date of acquisition. The following table summarizes the allocation of the aggregate purchase price to the fair value of the assets acquired and liabilities assumed:



Current assets	$	40,558
Property, plant and equipment	20,862
Other long-term assets	5,115
Identifiable intangible assets	5,600
Goodwill	21,070
Total assets acquired	93,205
Current liabilities assumed	(9,165		)
Long-term debt including capital leases assumed, excluding current installments	(17,604		)
Other liabilities	(711		)
Noncontrolling interest	(28,075		)
Total purchase price	$	37,650

In connection with its purchase of a controlling interest in NG Advantage, the Company assumed debt of $20,439 on a consolidated basis related to purchases of capital assets and working capital needs. Immediately after the Company's purchase of the controlling interest, $10,361 of such debt was paid with proceeds of the Company's investment in NG Advantage, and the related debt instruments were canceled.

Management allocated $5,600 of the purchase price to the identifiable intangible assets related to customer relationships and trade names that were acquired with the acquisition. The fair value of the identifiable intangible assets will be amortized on a straight-line basis over the estimated useful lives of such assets ranging from four to seven years. The excess of the purchase price over the fair value of net assets acquired was allocated to goodwill, which primarily represents additional market share available to the Company as a result of the acquisition, and is not deductible for income tax purposes.

Management determined the fair value of the noncontrolling interest to be $28,075 using a market approach and using inputs that included use of a comparable transaction to calculate the value of the noncontrolling interest adjusted for a control premium.

The results of NG Advantage's operations have been included in the Company's consolidated financial statements since October 14, 2014. The Company recorded a loss from the noncontrolling interest of $1,200, $1,216 and $1,571 for the years ended December 31, 2014, 2015 and 2016, respectively. The noncontrolling interest was $26,393 and $24,822 as of December 31, 2015 and 2016, respectively. For the year ended December 31, 2016, NG Advantage purchased assets for $1,550.

DCE and DCEMB

On September 4, 2014, Mavrix, LLC ("Mavrix"), a wholly owned subsidiary of the Company, sold to Cambrian Energy McCommas Bluff III LLC ("Cambrian") 19% of its then 70% interest in Dallas Clean Energy, LLC ("DCE"). On December 29, 2014, Mavrix entered into a Membership Interest Purchase Agreement (the "Agreement") with Cambrian, pursuant to which Mavrix sold to Cambrian its entire remaining 51% interest in DCE. DCE owns all of the equity interests in DCEMB, which owns a RNG extraction and processing project at the McCommas Bluff landfill in Dallas, Texas. As consideration for the sale of DCE, the Company, through Mavrix, received $6,992 in cash in September 2014, $40,588 in cash in December 2014 and $1,118 in cash in September 2015 due to the results of certain performance tests performed at the McCommas Bluffs project in accordance with the terms of the Agreement. The Company continues to have the right to market and sell biomethane produced at the McCommas Bluff project under its Redeem™ renewable natural gas vehicle fuel brand. The transaction resulted in a total gain of $12,935, comprised of $11,998 and $937 that was recorded in gain from sale of subsidiary in the Company's statements of operations for the years ended December 31, 2014 and 2015, respectively. Included in the determination of the total gain is goodwill that was allocated to the disposed business based on the relative fair values of the business disposed and the portion of the reporting unit that was retained.

The Company determined that the disposal did not meet the definition of a discontinued operation as the disposal did not represent a significant disposal nor was the disposal a strategic shift in the Company's strategy.

Restricted Cash

12 Months Ended

Dec. 31, 2016

Cash and Cash Equivalents [Abstract]

Restricted Cash

The Company classifies restricted cash as short-term and a current asset if the cash is expected to be used in operations within a year or to acquire a current asset. Otherwise, the restricted cash is classified as long-term. Restricted cash as of December 31, 2015 and 2016 consisted of the following:



	December 31, 2015		December 31, 2016
Short-term restricted cash:
Standby letters of credit	$	1,631	$	1,753
Canton Bonds (see Note 10)	2,609		3,665
Held in escrow	—		1,578
Total short-term restricted cash	$	4,240	$	6,996

Investments

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Investments

Available-for-sale securities are carried at fair value, inclusive of unrealized gains and losses. Unrealized gains and losses are included in other comprehensive income (loss) net of applicable income taxes. Gains or losses on sales of available-for-sale securities are recognized on the specific identification basis. All of the Company's short-term investments are classified as available-for-sale securities.

The Company reviews available-for-sale securities for other-than-temporary declines in fair value below their cost basis each quarter and whenever events or changes in circumstances indicate that the cost basis of an asset may not be recoverable. This evaluation is based on a number of factors, including the length of time and the extent to which the fair value has been below its cost basis and adverse conditions related specifically to the security, including any changes to the credit rating of the security. As of December 31, 2016, the Company believes its carrying values for its available-for-sale securities are properly recorded.

Short-term investments as of December 31, 2015 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	16,797	$	(7	)	$	16,790
Zero coupon bonds	500		(1		)	499
Corporate bonds	37,181		(77		)	37,104
Certificate of deposits	48,551		—			48,551
Total short-term investments	$	103,029	$	(85	)	$	102,944

Short-term investments as of December 31, 2016 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	8,791	$	(4	)	$	8,787
Corporate bonds	21,517		(7		)	21,510
Certificate of deposits	43,421		—			43,421
Total short-term investments	$	73,729	$	(11	)	$	73,718

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Fair Value Measurements

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value Measurements

The Company follows the authoritative guidance for fair value measurements with respect to assets and liabilities that are measured at fair value on a recurring basis and non-recurring basis. Under the standard, fair value is defined as the exit price, or the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants, as of the measurement date. The standard also establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs market participants would use in valuing the asset or liability developed based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the factors market participants would use in valuing the asset or liability developed based upon the best information available in the circumstances. The hierarchy consists of the following three levels: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs (other than quoted prices) that are observable for the asset or liability, either directly or indirectly; Level 3 inputs are unobservable inputs for the asset or liability. Categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.

As of December 31, 2016, the Company's financial instruments consisted of available-for-sale securities, debt instruments and liability-classified warrants. The Company’s available-for-sale securities are classified within Level 2 because they are valued using the most recent quoted prices for identical assets in markets that are not active and quoted prices for similar assets in active markets. The liability-classified warrants are classified within Level 3 because the Company uses the Black-Scholes option pricing model to estimate the fair value based on inputs that are not observable in any market. The fair values of the Company's debt instruments approximated their carrying values as of December 31, 2015 and 2016. See Note 10 for further information about the Company's debt instruments. There were no transfers of assets between Level 1, Level 2 or Level 3 of the fair value hierarchy as of as of December 31, 2015 and December 31, 2016, respectively.

The following tables provide information by level for assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2015 and December 31, 2016, respectively:



Description	Balance at December 31, 2015		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	16,790	$	—	$	16,790	$	—
Zero coupon bonds	499		—		499		—
Corporate bonds	37,104		—		37,104		—
Certificate of deposits	48,551		—		48,551		—
Liabilities:
Warrants(2)	632		—		—		632



Description	Balance at December 31, 2016		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	8,787	$	—	$	8,787	$	—
Corporate bonds	21,510		—		21,510		—
Certificate of deposits	43,421		—		43,421		—
Liabilities:
Warrants(2)	581		—		—		581

_______________________________________________________________________________


(1)	Included in short-term investments in the consolidated balance sheets. See Note 4 for further information.


(2)	Included in accrued liabilities and other long-term liabilities in the consolidated balance sheets.

The following tables provide a reconciliation of the beginning and ending balances of items measured at fair value on a recurring basis in the tables above that used significant unobservable inputs (Level 3).



Liabilities: Warrants	2015			2016
Beginning Balance	$	2,046		$	632
Gain included in earnings	(1,414		)	(51		)
Ending Balance	$	632		$	581

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Other Receivables

12 Months Ended

Dec. 31, 2016

Receivables [Abstract]

Other Receivables

Other receivables as of December 31, 2015 and 2016 consisted of the following:



	2015		2016
Loans to customers to finance vehicle purchases	$	10,531	$	7,416
Accrued customer billings	7,106		4,308
Fuel tax credits	40,730		6,358
Other	2,300		3,852
Total other receivables	$	60,667	$	21,934

Land, Property and Equipment

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Land, Property and Equipment

Land, property and equipment as of December 31, 2015 and 2016 consisted of the following:



	2015			2016
Land	$	2,858		$	2,858
LNG liquefaction plants	94,634			94,634
RNG plants	46,397			47,545
Station equipment	316,258			341,605
Trailers	50,414			54,985
Other equipment	83,687			93,118
Construction in progress	139,586			117,662
	733,834			752,407
Less accumulated depreciation	(217,510		)	(268,484		)
Total land, property and equipment, net	$	516,324		$	483,923

Included in the land, property and equipment are capitalized software costs of $22,886 and $25,728 as of December 31, 2015 and 2016, respectively. The accumulated amortization on the capitalized software costs is $13,793 and $17,237 as of December 31, 2015 and 2016, respectively. The Company recorded $2,993, $3,053 and $3,444 of amortization expense related to the capitalized software costs during the years ended December 31, 2014, 2015 and 2016, respectively.

As of December 31, 2015 and 2016, $5,955 and $4,053, respectively, are included in accounts payable and accrued liabilities balances, which amounts are related to purchases of property and equipment. These amounts are excluded from the consolidated statements of cash flows as they are non-cash investing activities.

Investment in Other Entities	12 Months Ended
Investment in Other Entities	Dec. 31, 2016
Investments, All Other Investments [Abstract]
Investment in Other Entities	Investment in Other Entities In November 2016, the Company, through its wholly owned subsidiary, Clean Energy Renewables ("Renewables"), entered into agreements to form joint ventures with Aria Energy Operating LLC ("Aria"), a developer of RNG production facilities, to develop RNG production facilities at a Republic Services landfill in Oklahoma City, Oklahoma and an Advanced Disposal landfill near Atlanta, Georgia. These joint ventures are referred to as the "RNG Ventures." Renewables and Aria each have a 50% ownership interest in the RNG Ventures and, subject to certain conditions, are each responsible for 50% of the costs of developing the RNG production facilities that are owned by the RNG Ventures. Additionally, Renewables has the exclusive right to purchase 100% of the RNG that will be produced by these facilities for the vehicle fuels market. The Company accounts for its interest in the RNG Ventures using the equity method of accounting as the Company has the ability to exercise significant influence over these operations. As of December 31, 2016, the Company had an investment balance of $833 in the RNG Ventures. On September 16, 2014, the Company formed a joint venture with Mansfield Ventures LLC (“Mansfield Ventures”) called Mansfield Clean Energy Partners LLC ("MCEP"), which is designed to provide natural gas fueling solutions to bulk fuel haulers in the United States. The Company and Mansfield Ventures each have a 50% ownership interest in MCEP. The Company accounts for its interest in MCEP using the equity method of accounting as the Company has the ability to exercise significant influence over MCEP's operations. The Company recorded a loss from this investment of $490, $815 and $22 for the years ended December 31, 2014, 2015 and 2016, respectively. Additionally, during the year ended December 31, 2016, the Company received a return of capital of $3,031 with no change in ownership interest. The Company has an investment balance of $4,695 and $1,642 as of December 31, 2015 and 2016, respectively.

Accrued Liabilities

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Accrued Liabilities

Accrued liabilities as of December 31, 2015 and 2016 consisted of the following:



	2015		2016
Accrued alternative fuels incentives (1)	15,651		9,840
Accrued employee benefits	3,042		4,317
Accrued interest	3,718		1,849
Accrued gas and equipment purchases	14,133		11,657
Accrued property and other taxes	5,344		4,572
Salaries and wages	9,537		12,293
Other	7,657		8,073
Total accrued liabilities	$	59,082	$	52,601

(1) Includes VETC and tradable RINs and LCFS Credits (discussed in Note 1) payable to third parties.

Debt

12 Months Ended

Dec. 31, 2016

Debt Disclosure [Abstract]

Debt

Debt and capital lease obligations as of December 31, 2015 and 2016 consisted of the following and are further discussed below:



				December 31, 2015
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes (1)	$	150,000		399			$	149,601
SLG Notes	145,000			38			$	144,962
5.25% Notes	250,000			3,985			$	246,015
Canton Bonds	10,910			514			$	10,396
Capital lease obligations	6,448			—			$	6,448
Other debt	10,056			328			$	9,728
Total debt and capital lease obligations	572,414			5,264			567,150
Less amounts due within one year	(150,129		)	(273		)	(149,856		)
Total long-term debt and capital lease obligations	$	422,285		$	4,991		$	417,294



				December 31, 2016
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes (1)	150,000			274			$	149,726
5.25% Notes	110,450			1,088			$	109,362
PlainsCapital Bank Credit Facility	23,500			—			23,500
Canton Bonds	9,520			373			$	9,147
Capital lease obligations	6,028			—			$	6,028
Other debt	14,850			237			$	14,613
Total debt and capital lease obligations	314,348			1,972			312,376
Less amounts due within one year	(6,126		)	(183		)	(5,943		)
Total long-term debt and capital lease obligations	$	308,222		$	1,789		$	306,433

(1) Includes 7.5% $65,000 in principal amount held by T. Boone Pickens ("Pickens") as of December 31, 2016, which is classified as “Long-term debt, related party” on the consolidated balance sheets. See the description below for additional information.

The following is a summary of the aggregate maturities of debt and capital lease obligations for each of the years ending December 31, as of December 31, 2016:



	2017		2018		2019		2020		2021		Thereafter
7.5% Notes (1)	—		50,000		50,000		50,000		—		—
5.25% Notes	—		110,450		—		—		—		—
PlainsCapital Bank Credit Facility	—		23,500		—		—		—		—
Canton Bonds	1,420		1,460		1,555		1,665		1,695		1,725
Capital lease obligations	1,890		1,569		1,568		462		265		274
Other debt	2,816		2,667		2,370		2,507		2,036		2,454
Total	$	6,126	$	189,646	$	55,493	$	54,634	$	3,996	$	4,453

7.5% Notes

On July 11, 2011, the Company entered into a loan agreement (the "CHK Agreement") with Chesapeake NG Ventures Corporation ("Chesapeake"), an indirect wholly owned subsidiary of Chesapeake Energy Corporation, whereby Chesapeake agreed to purchase from the Company up to $150,000 of debt securities pursuant to the issuance of three convertible promissory notes over a three-year period, each having a principal amount of $50,000 (each a "CHK Note" and collectively the "CHK Notes" and, together with the CHK Agreement and other transaction documents, the "CHK Loan Documents"). The first CHK Note was issued on July 11, 2011 and the second CHK Note was issued on July 10, 2012.

On June 14, 2013 (the "Transfer Date"), Pickens and Green Energy Investment Holdings, LLC, an affiliate of Leonard Green & Partners, L.P. (collectively, the "Buyers") and Chesapeake entered into a note purchase agreement ("Note Purchase Agreement") pursuant to which Chesapeake sold the outstanding CHK Notes (the "Sale") to the Buyers. Chesapeake assigned to the Buyers all of its right, title and interest under the CHK Loan Documents (the "Assignment") and each Buyer severally assumed all of the obligations of Chesapeake under the CHK Loan Documents arising after the Sale and the Assignment including, without limitation, the obligation to advance an additional $50,000 to the Company in June 2013 (the "Assumption"). The Company also entered into the Note Purchase Agreement for the purpose of consenting to the Sale, the Assignment and the Assumption.

Contemporaneously with the execution of the Note Purchase Agreement, the Company entered into a loan agreement with each Buyer (collectively, the "Amended Agreements"). The Amended Agreements have the same terms as the CHK Agreement, other than changes to reflect the new holders of the CHK Notes. Immediately following execution of the Amended Agreements, the Buyers delivered $50,000 to the Company in satisfaction of the funding requirement they had assumed from Chesapeake (the "2013 Advance"). In addition, the Company canceled the existing CHK Notes and issued replacement notes and the Company also issued notes to the Buyers in exchange for the 2013 Advance (the replacement notes and the notes issued in exchange for the 2013 Advance are referred to herein as the "7.5% Notes").

The 7.5% Notes have the same terms as the original CHK Notes, other than changes to reflect their different holders. They bear interest at the rate of 7.5% per annum and are convertible at the option of the holder into shares of the Company's common stock at a conversion price of $15.80 per share (the "7.5% Notes Conversion Price"). Upon written notice to the Company, each holder of a 7.5% Note has the right to exchange all or any portion of the principal and accrued and unpaid interest under its 7.5% Notes for shares of the Company's common stock at the 7.5% Notes Conversion Price.

Additionally, subject to certain restrictions, the Company can force conversion of each 7.5% Note into shares of its common stock if, following the second anniversary of the issuance of a 7.5% Note, such shares trade at a 40% premium to the 7.5% Notes Conversion Price for at least 20 trading days in any consecutive 30 trading day period.

The entire principal balance of each 7.5% Note is due and payable seven years following its issuance and the Company may repay each 7.5% Note at maturity in shares of its common stock (with a value determined by the per share volume weighted-average price for the 20 trading days prior to the maturity date) or cash. All of the shares issuable upon conversion of the 7.5% Notes have been registered for resale by their holders pursuant to a registration statement that has been filed with and declared effective by the Securities and Exchange Commission.

The Amended Agreements provide for customary events of default which, if any of them occurs, would permit or require the principal of, and accrued interest on, the 7.5% Notes to become, or to be declared, due and payable. No events of default under the 7.5% Notes have occurred as of December 31, 2016.

On August 27, 2013, Green Energy Investment Holdings, LLC transferred $5,000 in principal amount of the 7.5% Notes to certain third parties.

As a result of the foregoing transactions, as of December 31, 2016, (i) Pickens held 7.5% Notes in the aggregate principal amount of $65,000, (ii) Green Energy Investment Holdings, LLC held 7.5% Notes in the aggregate principal amount of $80,000 and (iii) other third parties held 7.5% Notes in the aggregate principal amount of $5,000.

SLG Notes

On August 24, 2011, the Company entered into convertible note purchase agreements (each, an "SLG Agreement" and collectively the "SLG Agreements") with each of Springleaf Investments Pte. Ltd., a wholly owned subsidiary of Temasek Holdings Pte. Ltd., Lionfish Investments Pte. Ltd., an investment vehicle managed by Seatown Holdings International Pte. Ltd., and Greenwich Asset Holding Ltd., a wholly owned subsidiary of RRJ Capital Master Fund I, L.P. (each, a "Purchaser" and collectively, the "Purchasers"), pursuant to which the Purchasers agreed to purchase from the Company $150,000 of 7.5% convertible promissory notes due in August 2016 (each a "SLG Note" and collectively the "SLG Notes"). The transaction closed and the SLG Notes were issued on August 30, 2011. On March 1, 2012, Springleaf Investments Pte Ltd. transferred $24,000 in principal amount of the SLG Notes to Baytree Investments (Mauritius) Pte Ltd.

On February 29, 2016, and pursuant to the consent of the holders of the SLG Notes, the Company prepaid in cash an aggregate of $60,000 in principal amount and $1,812 in accrued and unpaid interest owed under the SLG Notes.

On July 14, 2016, the Company entered into separate privately negotiated exchange agreements with each holder of an SLG Note to exchange the outstanding principal amount of the SLG Notes, totaling $85,000 for all SLG Notes and all accrued and unpaid interest thereon, totaling $248 for all SLG Notes, for an aggregate of 14,000,000 shares of the Company's common stock and $38,155 in cash. The value of the shares of the Company's common stock issued to the holders of the SLG Notes in the exchange has been excluded from the Company's consolidated statements of cash flows, as it is a non-cash financing activity. The Company recognized a loss of $891 for the year ended December 31, 2016 related to the exchange of the SLG Notes for the Company's common stock. The repurchased and exchanged SLG Notes have been surrendered and canceled in full and the Company has no further obligations under the SLG Notes.

5.25% Notes

In September 2013, the Company completed a private offering of $250,000 in principal amount of 5.25% Convertible Senior Notes due 2018 (the "5.25% Notes") and entered into an indenture governing the 5.25% Notes (the "Indenture").

The net proceeds from the sale of the 5.25% Notes after the payment of certain debt issuance costs of $7,805 were $242,195. The Company has used the net proceeds from the sale of the 5.25% Notes to fund capital expenditures and for general corporate purposes. The 5.25% Notes bear interest at a rate of 5.25% per annum, payable semi- annually in arrears on October 1 and April 1 of each year, beginning on April 1, 2014. The 5.25% Notes will mature on October 1, 2018, unless purchased, redeemed or converted prior to such date in accordance with their terms and the terms of the Indenture.

Holders may convert their 5.25% Notes, at their option, at any time prior to the close of business on the business day immediately preceding the maturity date of the 5.25% Notes. Upon conversion, the Company will deliver a number of shares of its common stock, per $1 principal amount of 5.25% Notes, equal to the conversion rate then in effect (together with a cash payment in lieu of any fractional shares). The initial conversion rate for the 5.25% Notes is 64.1026 shares of the Company's common stock per $1 principal amount of 5.25% Notes (which is equivalent to an initial conversion price of approximately $15.60 per share of the Company's common stock). The conversion rate is subject to adjustment upon the occurrence of certain specified events as described in the Indenture.

Upon the occurrence of certain corporate events prior to the maturity date of the 5.25% Notes, the Company will, in certain circumstances, in addition to delivering the number of shares of the Company's common stock deliverable upon conversion of the 5.25% Notes based on the conversion rate then in effect (together with a cash payment in lieu of any fractional shares), pay holders that convert their 5.25% Notes, a cash make-whole payment in an amount as described in the Indenture. The Company may, at its option, irrevocably elect to settle its obligation to pay any such make-whole payment in shares of its common stock instead of in cash. The amount of any make-whole payment, whether it is settled in cash or in shares of the Company's common stock upon the Company's election, will be determined based on the date on which the corporate event occurs or becomes effective and the stock price paid (or deemed to be paid) per share of the Company's common stock in the corporate event, as described in the Indenture.

The Company may not redeem the 5.25% Notes prior to October 5, 2016. On or after October 5, 2016, the Company may, at its option, redeem for cash all or any portion of the 5.25% Notes if the closing sale price of the Company's common stock for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which notice of redemption is provided, exceeds 160% of the conversion price on each applicable trading day. In the event of the Company's redemption of the 5.25% Notes, the redemption price will equal 100% of the principal amount of the 5.25% Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. No sinking fund is provided for in the 5.25% Notes.

If the Company undergoes a fundamental change (as defined in the Indenture) prior to the maturity date of the 5.25% Notes, subject to certain conditions as described in the Indenture, holders may require the Company to purchase, for cash, all or any portion of their 5.25% Notes at a repurchase price equal to 100% of the principal amount of the 5.25% Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change purchase date.

The Indenture contains customary events of default with customary cure periods, including, without limitation, failure to make required payments or deliveries of shares of the Company’s common stock when due under the Indenture, failure to comply with certain covenants under the Indenture, failure to pay when due or acceleration of certain other indebtedness of the Company or certain of its subsidiaries, and certain events of bankruptcy and insolvency of the Company or certain of its subsidiaries. The occurrence of an event of default under the Indenture will allow either the trustee or the holders of at least 25% in principal amount of the then-outstanding 5.25% Notes to accelerate, or upon an event of default arising from certain events of bankruptcy or insolvency of the Company, will automatically cause the acceleration of, all amounts due under the 5.25% Notes. No events of default under the 5.25% Notes had occurred as of December 31, 2016.

The 5.25% Notes are senior unsecured obligations of the Company and rank senior in right of payment to the Company's future indebtedness that is expressly subordinated in right of payment to the 5.25% Notes; equal in right of payment to the Company's unsecured indebtedness that is not so subordinated; effectively junior to any of the Company's secured indebtedness to the extent of the value of the assets securing such indebtedness; and structurally junior to all indebtedness (including trade payables) of the Company's subsidiaries.

For the year ended December 31, 2016, the Company paid an aggregate of $84,344 in cash to repurchase and retire $114,550 in aggregate principal amount of the 5.25% Notes, together with $1,546 in accrued and unpaid interest thereon. Additionally, pursuant to a privately negotiated exchange agreement with certain holders of the 5.25% Notes, on May 4, 2016, the Company issued 6,265,829 shares of its common stock in exchange for an aggregate principal amount of $25,000 of 5.25% Notes held by such holders and accrued and unpaid interest thereon. The value of the shares of the Company's common stock issued to the holders of the 5.25% Notes in the exchange has been excluded from the Company's consolidated statements of cash flows as it is a non-cash financing activity. The Company's repurchase and exchange of 5.25% Notes for the year ended December 31, 2016 resulted in a total gain of $35,239 recorded during the period. All repurchased and exchanged 5.25% Notes have been surrendered to the trustee for such notes and canceled in full and the Company has no further obligations under such notes.

PlainsCapital Bank Credit Facility

On February 29, 2016, the Company entered into a Loan and Security Agreement (“LSA”) with PlainsCapital Bank (“Plains”), pursuant to which Plains agreed to lend the Company up to $50,000 on a revolving basis from time to time for a term of one year (the “Credit Facility”). Simultaneously, the Company drew down $50,000 under this Credit Facility, which the Company repaid in full on August 31, 2016. On October 31, 2016 the LSA was amended solely to extend the Credit Facility's maturity date from February 28, 2017 to September 30, 2018. On December 22, 2016, the Company drew down $23,500 under the Credit Facility, which remained outstanding on December 31, 2016. The Credit Facility is evidenced by a promissory note the Company issued on February 29, 2016 in favor of Plains (the “Plains Note”). Interest on the Plains Note is payable monthly and accrues at a rate equal to the greater of (i) the then-current LIBOR rate plus 2.30% or (ii) 2.70%. As collateral security for the prompt payment in full when due of the Company's obligations to Plains under the LSA and the Plains Note, the Company pledged to and granted Plains a security interest in all of its right, title and interest in the cash and corporate and municipal bonds rated AAA, AA or A by Standard & Poor’s Rating Services that the Company holds in an account at Plains. In connection with such pledge and security interest granted under the Credit Facility, on February 29, 2016, the Company entered into a Pledged Account Agreement with Plains and PlainsCapital Bank - Wealth Management and Trust (the “Pledge Agreement” and collectively with the LSA and the Plains Note, the “Plains Loan Documents”).The Plains Loan Documents include certain covenants of the Company and also provide for customary events of default, which, if any of them occurs, would permit or require, among other things, the principal of, and accrued interest on, the Credit Facility to become, or to be declared, due and payable. Events of default under the Plains Loan Documents include, among others, the occurrence of certain bankruptcy events, the failure to make payments when due under the Plains Note and the transfer or disposal of the collateral under the LSA. The Company had $23,500 outstanding under the Credit Facility as of December 31, 2016 and no events of default under the Plains Loan Documents had occurred as of December 31, 2016.

Canton Bonds

On March 19, 2014, Canton Renewables LLC ("Canton"), a wholly owned subsidiary of the Company, completed the issuance of Solid Waste Facility Limited Obligation Revenue Bonds (Canton Renewables, LLC—Sauk Trail Hills Project) Series 2014 in the aggregate principal amount of $12,400 (the "Canton Bonds").

The Canton Bonds were issued by the Michigan Strategic Fund (the "Issuer") and the proceeds of the issuance were loaned by the Issuer to Canton pursuant to a loan agreement that became effective on March 19, 2014 (the "Loan Agreement"). The Canton Bonds are expected to be repaid from revenue generated by Canton from the sale of RNG and are secured by the revenue and assets of Canton. The Canton Bond repayments will be amortized through July 1, 2022, the average coupon interest rate on the Canton Bonds is 6.6%, and all but $1,000 of the principal amount of the Canton Bonds is non-recourse to Canton's parent companies, including the Company.

Canton used the Canton Bond proceeds primarily to (i) refinance the cost of constructing and equipping its RNG extraction and production project in Canton, Michigan and (ii) pay a portion of the costs associated with the issuance of the Canton Bonds. The refinancing described in the prior sentence was accomplished through distributions to Canton's direct and indirect parent companies who provided the financing for the RNG production facility, and such companies have used such distributions to finance construction of additional RNG extraction and processing projects and for working capital purposes.

The Loan Agreement contains customary events of default, with customary cure periods, including without limitation, failure to make required payments when due under the Loan Agreement, failure to comply with certain covenants under the Loan Agreement, certain events of bankruptcy and insolvency of Canton, and the existence of an event of default under the indenture governing the Canton Bonds that was entered into between the Issuer and The Bank of New York Mellon Trust Company, N.A., as trustee. The occurrence of an event of default under the Loan Agreement will allow the Issuer or the trustee to, among other things, accelerate all amounts due under the Loan Agreement. No events of default under the Loan Agreement have occurred as of December 31, 2016.

Other Debt

The Company has other debt due at various dates through 2023 bearing interest at rates up to 21.5% and with a weighted average interest rate of 6.35% and 5.00% as of December 31, 2015 and 2016, respectively.

December 2015 Termination of GE Credit Agreement

On November 7, 2012, the Company, through two wholly owned subsidiaries (the "Borrowers"), entered into a credit agreement ("Credit Agreement") with General Electric Capital Corporation ("GE"). Pursuant to the Credit Agreement, GE agreed to loan to the Borrowers up to an aggregate of $200,000 to finance the development, construction and operation of two LNG plants (individually a "Project" and together the "Projects").

The Credit Agreement included a commitment fee on the unutilized loan amounts of 0.5% per annum, which was $1,014 for each of the years ended December 31, 2014 and 2015 and was charged to interest expense in the consolidated statements of operations.

Concurrently with the execution of the Credit Agreement, the Company issued to GE a warrant to purchase up to 5,000,000 shares of its common stock at a price of $0.01 per share (the "GE Warrant"). See Note 12 for further information.

On December 29, 2014, the Borrowers and GE entered into an amendment to the Credit Agreement providing, among other things, that (i) the Credit Agreement would terminate if the initial loans under the Credit Agreement (collectively, "Loans") for the Projects were not made prior to December 31, 2016 (rather than December 31, 2015, as the Credit Agreement originally provided), (ii) each Project was required to be completed by the earlier of (a) the date that is 30 months after the funding of the initial Loans with respect to such Project and (b) December 31, 2018 (rather than December 31, 2016, as the Credit Agreement originally provided), and (iii) prior to the funding of the Loans, the Borrowers were required to enter into agreements with GE Oil & Gas, Inc. relating to the purchase of equipment for the Projects.

On December 31, 2015, the Company terminated the Credit Agreement and all related documents except for the GE Warrant, which effectively became exercisable for only 1,000,000 shares because the vesting conditions relating to the other 4,000,000 shares subject to the GE Warrant could not occur following such termination. See Note 12 for further information about the GE Warrant. No amounts had been borrowed by the Borrowers under the Credit Agreement as of its termination. As a result of the termination of the Credit Agreement, all related unamortized deferred financing costs that were to be amortized to interest expense in future periods have been removed from the balance sheet and a non-cash charge totaling $54,925 was recorded in interest expense in the fourth quarter of 2015.

December 2014 Termination of Mavrix Note

On April 25, 2013, Mavrix entered into a note purchase agreement (the “NPA”) with Massachusetts Mutual Life Insurance (the “Note Purchaser”) and issued to the Note Purchaser a secured multi draw promissory note (the “Mavrix Note”) in the maximum aggregate principal amount of $30,000. The Note Purchaser funded $15,000 under the Mavrix Note during 2013. In connection with the Company’s sale of its interests in DCE and DCEMB, on December 29, 2014, Mavrix paid $13,594 to the Note Purchaser as payment in full of all outstanding indebtedness under the NPA and the Mavrix Note. Such amount includes approximately $750 as payment of an early termination fee required pursuant to the terms of the NPA and the Mavrix Note. Concurrently with such payment, the NPA, the Mavrix Note and all other documents related thereto were terminated in full.

Derivative Transactions	12 Months Ended
Derivative Transactions	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Transactions	Derivative Transactions The Company had no commodity futures contracts or forward exchange contracts outstanding during the years ended December 31, 2015 and 2016, respectively. The Company marks-to-market its open futures positions and forward exchange contracts at the end of each period and records the net unrealized gain or loss during the period in derivative (gains) losses in the consolidated statements of operations or in accumulated other comprehensive income (loss) in the consolidated balance sheets in accordance with the applicable accounting guidance. During the year ended December 31, 2014, the Company recorded unrealized gains of $108 in other comprehensive income (loss) related to its futures contracts and recognized a loss of $65 in cost of sales in the accompanying consolidated statements of operations related to its futures contracts that were settled during the year. This amount was reclassified from accumulated other comprehensive income (loss).

- Definition

The entire disclosure for derivative instruments and hedging activities including, but not limited to, risk management strategies, non-hedging derivative instruments, assets, liabilities, revenue and expenses, and methodologies and assumptions used in determining the amounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6441202&loc=d3e80720-113993

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5618551-113959

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959

Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4E
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624181-113959

Reference 17: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

Reference 18: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41638-113959

Reference 19: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(n))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Stockholders' Equity

12 Months Ended

Dec. 31, 2016

Stockholders' Equity Note [Abstract]

Stockholders' Equity

Authorized Shares

The Company's certificate of incorporation authorizes the issuance of two classes of capital stock designated as common stock and preferred stock, each having $0.0001 par value per share. As of December 31, 2016, the Company was authorized to issue 225,000,000 shares, of which 224,000,000 shares are designated common stock and 1,000,000 shares are designated preferred stock.

Dividend Provisions

The Company did not declare or pay any dividends during the years ended December 31, 2014, 2015 or 2016.

Voting Rights

Each holder of common stock has the right to one vote per share owned on matters presented for stockholder action.

Issuance of Common Stock and Warrants

Series I Warrants

In November 2008, the Company issued to certain investors 4,419,192 Series I Warrants to purchase up to 3,314,394 shares of common stock. The Series I Warrants became exercisable beginning six months from the date of issuance, had a term of seven years from the date they became exercisable, and carried an exercise price of $12.54 per share. All outstanding Series I Warrants expired in April 2016.

GE Warrant

Concurrently with the execution of the Credit Agreement on November 7, 2012, the Company issued to GE the GE Warrant, which entitled GE to purchase up to 5,000,000 shares of the Company's common stock at a price per share of $0.01. The Company terminated the Credit Agreement on December 31, 2015 and as a result, 4,000,000 shares subject to the GE Warrant could not become exercisable because the vesting conditions relating to these shares could not occur following such termination. On October 4, 2016, the holders of the GE Warrant exercised the warrant to purchase the 1,000,000 shares of common stock that were vested and exercisable thereunder pursuant to the cashless exercise provisions thereof, which resulted in the Company's issuance of 997,740 shares of common stock to such holders. Following such exercise, the GE Warrant has been surrendered and canceled in full and the Company has no further obligations under the GE Warrant.

The Company measured the fair value of the original 5,000,000 shares subject to the GE Warrant at $56,158 and recorded the amount in additional paid-in-capital and other long-term assets as a deferred financing cost. The fair value of the 1,000,000 shares that remained exercisable under the GE Warrant following the termination of the Credit Agreement were being amortized over the estimated term of the Credit Agreement on the straight-line basis. The issuance of the GE Warrant is not included in the consolidated statements of cash flows as it is a non-cash financing activity.

In connection with the termination of the Credit Agreement, all related unamortized deferred financing costs that were to be amortized to interest expense in future periods have been eliminated from the consolidated balance sheets in full through a non-cash charge to the consolidated statement of operations, reported in interest expense, of $54,925 in the fourth quarter of 2015. See Note 10 for further information.

At-The-Market Offering Program

On November 11, 2015, the Company entered into an equity distribution agreement with Citigroup Global Markets Inc. ("Citigroup") as sales agent and/or principal, pursuant to which the Company may issue and sell, from time to time, through or to Citigroup, shares of the Company's common stock having an aggregate offering price of up to $75,000 in an “at-the-market” offering program (the “ATM Program”). On September 9, 2016, the Company entered into an amended and restated equity distribution agreement with Citigroup for the primary purpose of increasing from $75,000 to $110,000, the aggregate offering price of shares of common stock available for issuance and sale in the ATM Program. On December 21, 2016, the Company entered into a second amended and restated equity distribution agreement with Citigroup, for the primary purpose of increasing from $110,000 to $200,000, the aggregate offering price of shares of common stock available for issuance and sale in the ATM program.

The following table summarizes the activity under the ATM Program for the periods presented:



	Year ended December 31,		Year ended December 31,
(in 000s, except for per-share amounts)	2015		2016
Gross proceeds	$	6,943	$	103,591
Fees and issuance costs	$	493	$	2,612
Net proceeds	$	6,450	$	100,979
Shares issued	1,561,902		31,064,434

Other

As of December 31, 2016, third parties held outstanding warrants, which expire in 2020, to purchase equity interests in NG Advantage. Such warrants allow the purchase of up to 127,200 NG Advantage common units and are accounted for as liability-classified warrants. The fair value was $561 and $581 as of December 31, 2015 and 2016, respectively (see Note 5 for additional information) and the gain (loss) from the change in fair value was $0, $69 and $(21) for the years ended December 31, 2014, 2015 and 2016, respectively.

Stock-Based Compensation

The following table summarizes the compensation expense and related income tax benefit related to the Company's stock-based compensation arrangements recognized in the consolidated statements of operations:



	Years Ended December 31,
	2014		2015		2016
Stock-based compensation expense, net of $0 tax in 2014, 2015 and 2016	$	11,514	$	10,779	$	8,092

Equity Incentive Plans

In December 2002, the Company adopted its 2002 Stock Option Plan ("2002 Plan").

In December 2006, the Company adopted its 2006 Equity Incentive Plan ("2006 Plan"), which became effective on May 24, 2007, the date the Company completed its initial public offering of common stock. The 2002 Plan became unavailable for new awards upon the effectiveness of the 2006 Plan. Unissued awards under the 2002 Plan are available for future grant under the 2006 Plan. If any outstanding option under the 2002 Plan expires or is canceled, the shares allocable to the unexercised portion of that option will be added to the share reserve under the 2006 Plan and will be available for grant under the 2006 Plan.

In May 2016, the Company adopted its 2016 Performance Incentive Plan ("2016 Plan"), which became effective on May 26, 2016, the date of approval of the 2016 Plan by the Company's stockholders. The 2006 Plan became unavailable for new awards upon the effectiveness of the 2016 Plan. Unissued awards under the 2002 and 2006 Plans are not available for future grant under the 2016 Plan. If any outstanding award under the 2002 and 2006 Plans expires or is canceled, the shares allocable to the unexercised portion of that award will be added to the share reserve under the 2016 Plan and will be available for grant under the 2016 Plan. As of December 31, 2016, the Company had 6,112,582 shares available for future grant under the 2016 Plan.

Stock Options

The following table summarizes the Company's stock option activity:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding as of December 31, 2013	11,526,998		$	11.79
Granted	957,000		10.23
Exercised	(468,279	)	3.78
Forfeited or Expired	(529,418	)	13.40
Options Outstanding as of December 31, 2014	11,486,301		$	11.91
Granted	1,415,200		5.39
Exercised	(608,279	)	2.96
Forfeited or Expired	(805,284	)	14.04
Options Outstanding as of December 31, 2015	11,487,938		$	11.44
Granted	284,750		3.63
Exercised	—		—
Forfeited or Expired	(304,892	)	11.30
Options Outstanding as of December 31, 2016	11,467,796		$	11.25	3.99	—
Options Exercisable as of December 31, 2016	10,176,324		$	11.96	3.41	—
Options Vested and Expected to Vest as of December 31, 2016	11,351,637		$	11.30	3.88	—

As of December 31, 2016, there was $2,794 of total unrecognized compensation cost related to unvested shares underlying outstanding stock options. That cost is expected to be expensed over a remaining weighted average period of 1.68 years. The total fair value of shares vested during the year ended December 31, 2016 was $2,544.

The intrinsic value of all stock options exercised during the year ended December 31, 2014, 2015 and 2016 was $2,568, $2,197 and $0 respectively.

The fair value of each stock option granted was estimated as of the date of grant using the Black-Scholes option pricing model and using the following assumptions:



	Years Ended December 31,
	2014	2015	2016
Dividend yield	0.0%	0.0%	0.0%
Expected volatility	52.3% to 67.0%	59.2% to 72.0%	61.1% to 70.8%
Risk-free interest rate	1.1% to 1.8%	1.7% to 1.8%	1.2% to 2.0%
Expected life in years	6.0	6.0	6.0

The weighted-average grant date fair values of stock options granted during the years ended December 31, 2014, 2015 and 2016, were $5.32, $3.29 and $2.30, respectively. The volatility amounts used were estimated based on the Company's historical and implied volatility of its traded options. The expected lives used were based on historical exercise periods and the Company's anticipated exercise periods for its outstanding stock options. The risk free interest rates used were based on the U.S. Treasury yield curve for the expected life of the stock options at the time of grant. The Company recorded $7,286, $5,195 and $2,561 of stock option expense during the years ended December 31, 2014, 2015 and 2016, respectively. The Company has not recorded any tax benefit related to its stock option expense.

Market-Based Performance Restricted Stock Units

The Company granted 2,034,500 market-based performance restricted stock units ("Market-Based RSUs") to certain key employees during 2012 and 2014. A holder of Market-Based RSUs will receive one share of the Company's common stock for each Market-Based RSU held if (x) between two years and four years from the date of grant of the Market-Based RSU, the closing price of the Company's common stock equals or exceeds, for twenty consecutive trading days, 135% of the closing price of the Company's common stock on the Market-Based RSU grant date (the "Stock Price Condition") and (y) the holder is employed by the Company at the time the Stock Price Condition is satisfied. If the Stock Price Condition is not satisfied prior to four years from the date of grant, the Market-Based RSUs are automatically forfeited. As a result, as of December 31, 2016, Market-Based RSUs granted in January and May 2012 and entitling the holders to receive 1,605,500 shares of the Company’s common stock had been forfeited for failure to satisfy the applicable Stock Price Condition.

The Market-Based RSUs are subject to the terms and conditions of the 2006 Plan and a Notice of Grant of Restricted Stock Unit and Restricted Stock Unit Agreement.

The following table summarizes the Company's Market-Based RSU activity:



	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
RSU Outstanding as of December 31, 2013	1,545,000		$	11.42
Granted	489,500		8.26
Vested	—		—
Forfeited or Expired	(265,500	)	10.62
RSU Outstanding as of December 31, 2014	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding as of December 31, 2015	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	(1,340,000	)	11.44
RSU Outstanding and Unvested as of December 31, 2016	429,000		$	8.26	1.09
RSU Expected to Vest as of December 31, 2016	—		—		0.00

As of December 31, 2016, there was $0 of total unrecognized compensation cost related to unvested shares underlying outstanding Market-Based RSUs.

The Company recorded $2,556, $1,770 and $169 of expense during the years ended December 31, 2014, 2015 and 2016, respectively, related to the Market-Based RSUs. The Company has not recorded any tax benefit related to its Market-Based RSU expense.

Service-Based Restricted Stock Units

The Company has granted service-based restricted stock units ("Service-Based RSUs") to key employees that vest annually over the three years following the date of grant at a rate of 34%, 33% and 33%, respectively, if the holder is in service to the Company at each vesting date. The Service-Based RSUs are subject to the terms and conditions of the 2006 and 2016 Plans and a Notice of Grant of Restricted Stock Unit and Restricted Stock Unit Agreement.

The following table summarizes the Company's Service-Based RSU activity:



	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
RSU Outstanding as of December 31, 2013	45,836		$	13.09
Granted	792,500		5.54
Vested	(15,584	)	13.09
Forfeited or expired	—		—
RSU Outstanding as of December 31, 2014	822,752		$	5.82
Granted	1,167,750		5.38
Vested	(283,726	)	5.94
Forfeited or expired	(56,000	)	5.57
RSU Outstanding as of December 31, 2015	1,650,776		$	5.50
Granted	850,125		3.63
Vested	(726,687	)	5.53
Forfeited or expired	(130,910	)	4.91
RSU Outstanding and Unvested as of December 31, 2016	1,643,304		$	4.56	1.19
RSU Expected to Vest as of December 31, 2016	1,545,222		$	4.56	0.99

As of December 31, 2016, there was $5,799 of total unrecognized compensation cost related to unvested shares underlying outstanding Service-Based RSUs. That cost is expected to be expensed over a remaining weighted average period of 1.21 years.

The Company recorded $365, $2,622 and $4,395 of expense during the years ended December 31, 2014, 2015 and 2016, respectively, related to the Service-Based RSUs. The Company has not recorded any tax benefit related to its Service-Based RSU expense.

The fair value of each Service-Based RSU granted during the year ended December 31, 2016 was estimated using the closing stock price of the Company's common stock on the date of grant.

Employee Stock Purchase Plan

On May 7, 2013, the Company adopted an employee stock purchase plan (the "ESPP"), pursuant to which eligible employees may purchase shares of the Company's common stock at 85% of the fair market value of the common stock on the last trading day of two consecutive, non-concurrent offering periods each year. The Company has reserved 2,500,000 shares of its common stock for issuance under the ESPP, and the first offering period under the ESPP commenced on September 1, 2013.

The Company recorded $67, $50 and $51 of expense related to the ESPP during the years ended December 31, 2014, 2015 and 2016, respectively. The Company has not recorded any tax benefits related to its ESPP expense. As of December 31, 2016, the Company had issued an aggregate of 199,884 shares pursuant to the ESPP.

Non-Qualified Non-Public Subsidiary Unit Options

In September 2013, the Company's wholly owned subsidiary Renewables adopted a unit option plan (the "Clean Energy Renewables Plan"). 150,000 Class B units representing membership interests in Renewables were initially reserved for issuance under the Clean Energy Renewables Plan.

The following table summarizes Renewables' unit option activity:



	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding as of December 31, 2013	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding as of December 31, 2014	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	(7,000	)	40.80
Options Outstanding as of December 31, 2015	108,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding and Exercisable as of December 31, 2016	108,000		$	40.80	6.72	$	94.62

As of December 31, 2016, there was $0 of total unrecognized compensation cost related to unvested units underlying outstanding unit options.

The grant date fair value of unit options granted in September 2013 was $31.65, which was determined contemporaneously with the unit option grants. The volatility amount used was estimated based on the historical volatility of a certain peer group of Renewables for a period commensurate with the expected life of the unit options granted. The expected life used was Renewables' anticipated exercise periods for its outstanding unit options. The risk free interest rate used was based on the U.S. Treasury yield curve for the expected life of the unit options at the time of grant. Renewables recorded $1,240, $1,115 and $803 of unit option expense during the years ended December 31, 2014, 2015 and 2016, respectively. Renewables has not recorded any tax benefit related to its unit option expense.

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

The components of income (loss) before income taxes for the years ended December 31, 2014, 2015 and 2016 are as follows:



	2014			2015			2016
U.S.	$	(64,913	)	$	(111,437	)	$	7,150
Foreign	(24,871		)	(22,407		)	(19,535		)
Total loss before income taxes	$	(89,784	)	$	(133,844	)	$	(12,385	)

The provision for income taxes consists of the following:



	2014			2015		2016
Current:
Federal	$	190		$	9	$	(226	)
State	238			248		93
Foreign	1,017			912		567
Total current	1,445			1,169		434
Deferred:
Federal	29			337		478
State	(10		)	71		75
Foreign	(389		)	37		352
Total deferred	(370		)	445		905
Total	$	1,075		$	1,614	$	1,339

Income tax expense for the years ended December 31, 2014, 2015 and 2016 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following:



	2014			2015			2016
Computed expected tax (benefit)	$	(30,415	)	$	(46,846	)	$	(4,335	)
Nondeductible expenses	10,690			24,998			5,971
Tax rate differential on foreign earnings	5,733			3,701			720
Tax credits	(8,286		)	(9,988		)	(9,331		)
Other	(1,121		)	(372		)	833
Change in valuation allowance	24,474			30,121			7,481
Total tax expense	$	1,075		$	1,614		$	1,339

During the year ended December 31, 2014, federal tax legislation enacted VETC through December 31, 2014 with retroactive effect to January 1, 2014, and during the year ended December 31, 2015, federal tax legislation enacted VETC through December 31, 2016 with retroactive effect to January 1, 2015. Additionally, in 2013 federal tax guidance was issued that clarified that the VETC in excess of the Company's fuel tax obligation, which is collected from customers, can be excluded from taxable income. The Company recorded a federal tax benefit of $8,221, $9,298 and $9,112 related to the exclusion of VETC associated with 2014, 2015 and 2016 fuel sales in excess of its fuel tax obligation, respectively. These amounts increased the Company's deferred tax asset attributed to its federal net operating loss carryforwards and the Company's deferred tax asset valuation allowance.

Deferred tax assets and liabilities result from differences between the financial statement carrying amounts and the tax bases of existing assets and liabilities. The tax effect of temporary differences that give rise to deferred tax assets and liabilities as of December 31, 2015 and 2016 are as follows:



	2015			2016
Deferred tax assets:
Accrued expenses	$	4,512		$	4,566
Sales-type leases	154			55
Alternative minimum tax and general business credits	5,780			6,137
Stock option expense	23,113			26,154
Other	2,283			1,168
Loss carryforwards	174,157			181,884
Total deferred tax assets	209,999			219,964
Less valuation allowance	(189,203		)	(195,968		)
Net deferred tax assets	20,796			23,996
Deferred tax liabilities:
Depreciation and amortization	(17,398		)	(19,364		)
Goodwill	(4,600		)	(5,599		)
Partnership income	(293		)	(1,432		)
Total deferred tax liabilities	(22,291		)	(26,395		)
Net deferred tax liabilities	$	(1,495	)	$	(2,399	)

As of December 31, 2016, the Company had federal, state and foreign net operating loss carryforwards of approximately $447,788, $329,058 and $79,148, respectively. The Company's federal, state and foreign net operating loss carryforwards will, if not utilized, expire beginning in 2026, 2017 and 2028, respectively. The Company also has federal tax credit carryforwards of $5,907 that will expire beginning in 2026. Due to the change of ownership provisions of Internal Revenue Code Section 382, utilization of a portion of the Company's net operating loss and tax credit carryforwards may be limited in future periods.

In assessing the realizability of the net deferred tax assets, management considers whether it is more likely than not that some or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers projected future taxable income and tax planning strategies in making this assessment. As of December 31, 2015 and 2016, the Company provided a valuation allowance of $189,203, and $195,968, respectively, to reduce the net deferred tax assets due to uncertainty surrounding the realizability of these assets. The net increase in the valuation allowance for the years ended December 31, 2015 and 2016 was $28,767, and $6,765, respectfully. The changes in the valuation allowance were primarily attributable to operating losses incurred in certain jurisdictions for which a full valuation allowance was established.

As of December 31, 2016, the Company has not provided deferred U.S. income taxes or foreign withholding taxes on temporary differences of approximately $4,568 resulting from earnings of certain non-U.S. subsidiaries which are permanently reinvested outside the United States. Unrecognized deferred taxes on remittance of these funds are not expected to be material.

The Company does not recognize the impact of a tax position in its financial statements unless the position is more likely than not to be sustained, based on the technical merits of the position. The Company has unrecognized tax benefits of $49,602 at December 31, 2016 including $692 of tax benefits that, if recognized, would reduce the Company's annual effective tax rate. The remaining $48,910, if recognized, would not result in a tax benefit since it would be fully offset with a valuation allowance.

The following is a tabular reconciliation of the total amounts of unrecognized tax benefits for the years ended December 31, 2014, 2015 and 2016:



Unrecognized tax benefit—December 31, 2014	$	21,974
Gross increases—tax positions in current year	5,523
Gross increases—tax positions in prior years	—
Unrecognized tax benefit—December 31, 2015	27,497
Gross increases—tax positions in current year	4,556
Gross increases—tax positions in prior years	17,549
Unrecognized tax benefit—December 31, 2016	$	49,602

The increase in the Company's unrecognized tax benefits during the years ended December 31, 2015 and 2016 is primarily attributable to the portion of VETC revenue that was offset by the fuel tax the Company collected from its customers as an unrecognized tax benefit during 2015 and 2016. The Company believes the portion of VETC revenue that is offset by the fuel tax the Company collects from its customers can be excluded from taxable income, although the ultimate outcome of this tax position is uncertain.

The Company increased its reserve for unrecognized tax positions in the year ended December 31, 2016. As unrecognized tax positions are not recognized for financial reporting purposes, this position does not have an impact on the balance sheet, statement of operations or statement of cash flows. If this position were to be sustained, then there would be an increase in the Company's deferred tax assets attributed to its federal and state net operating loss carryforwards, as well as an increase to the amount of the Company's deferred tax asset valuation allowance. The increase in the Company's reserve for unrecognized tax positions was attributable to the write-off of unamortized debt issuance costs resulting from the Company's termination of its Credit Agreement with GE on December 15, 2015. Although the ultimate outcome of this tax position is uncertain, the Company believes that this non-cash charge can be deducted in determining its U.S. taxable income for the year ended December 31, 2015.

FASB authoritative guidance requires the Company to accrue interest and penalties where there is an underpayment of taxes based on the Company's best estimate of the amount ultimately to be paid. The Company's policy is to recognize interest accrued related to unrecognized tax benefits and penalties as income tax expense. In addition to the unrecognized tax benefits noted above, the Company accrued $178 and $241 of interest expense as of December 31, 2015 and 2016, respectively. The Company recognized interest expense related to uncertain tax positions of $54, $58 and $62 for the years ended December 31, 2014, 2015 and 2016, respectively.

The Company is subject to taxation in the United States and various states and foreign jurisdictions. The Company's tax years for 2012 through 2016 are subject to examination by various tax authorities. While the Company is no longer subject to U.S. examination for years before 2013, and for state tax examinations for years before 2012, taxing authorities can adjust the net operating losses that arose in earlier years if and when the net operating losses reduce future income.

A number of years may elapse before an uncertain tax position is finally resolved. It is often difficult to predict the final outcome or the timing of resolution of an uncertain tax position, but the Company believes that its reserves for income taxes reflect the most probable outcomes. The Company adjusts the reserve, as well as the related interest and penalties, in light of changing facts and circumstances. The amount of penalties accrued is immaterial. Settlement of any particular position would usually require the use of cash and result in the reduction of the related reserve, or there could be a change in the amount of the Company's net operating loss. The resolution of a matter would be recognized as an adjustment to the provision for income taxes at the effective tax rate in the period of resolution. The Company does not expect a significant increase or decrease in its uncertain tax positions within the next twelve months.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Environmental Matters

The Company is subject to federal, state, local and foreign environmental laws and regulations. The Company does not anticipate any expenditures to comply with such laws and regulations that would have a material impact on the Company's consolidated financial position, results of operations or liquidity. The Company believes that its operations comply, in all material respects, with applicable federal, state, local and foreign environmental laws and regulations.

Litigation, Claims and Contingencies

The Company may become party to various legal actions in the ordinary course of its business. The Company is also subject to audit by tax authorities for varying periods in various federal, state, local and foreign tax jurisdictions, and disputes may arise during the course of these audits. It is impossible to determine the liabilities that the Company may incur resulting from any of these lawsuits and proceedings, audits, commitments, contingencies and related matters or the timing of these liabilities, if any. If these matters were to ultimately be resolved unfavorably it is possible that such an outcome could have a material adverse effect upon the Company's consolidated financial position, results of operations or liquidity. The Company, however, does not anticipate such an outcome and believes that the ultimate resolution of these matters will not have a material adverse effect on the Company's consolidated financial position, results of operations or liquidity.

Operating Lease Commitments

The Company leases facilities, including the land for its LNG production plant in Boron, California and certain equipment under noncancelable operating leases expiring at various dates through 2038. The following schedule represents the future minimum lease obligations for all noncancelable operating leases as of December 31, 2016:



Fiscal year:
2017	$	8,055
2018	6,685
2019	6,287
2020	5,575
2021	4,195
Thereafter	17,801
Total future minimum lease payments	$	48,598

Rent expense, including variable rent, totaled $10,140, $8,629, and $11,058 for the years ended December 31, 2014, 2015 and 2016, respectively.

Long-Term Take-or-Pay Natural Gas Supply Contracts

In October 2007, the Company entered into an LNG supply contract with Desert Gas Services (formerly known as Spectrum Energy Services, LLC) ("DGS") to purchase LNG, on a take-or-pay basis, starting in March 2010 and expiring in March 2020. For the years ended December 31, 2014, 2015 and 2016, the Company paid approximately $14,267, $11,852, and $9,692 respectively, under this contract. As of December 31, 2016, the fixed commitments under this contract totaled approximately $3,614, $3,614, $3,614 and $901 for the years ending December 31, 2017, 2018, 2019 and 2020 respectively.

During 2015, the Company entered into a CNG supply contract with Jacksonville Transit Authority ("JTA") to purchase CNG, on a take-or-pay basis, starting in January 2016 and expiring in December 2020. As of December 31, 2016, the fixed commitments under the JTA contract totaled approximately $203, $313, $429 and $548 for the years ending December 31, 2017, 2018, 2019 and 2020, respectively.

Capitalized Lease Obligation and Receivables

12 Months Ended

Dec. 31, 2016

Leases [Abstract]

Capitalized Lease Obligation and Receivables

The Company leases equipment under capital leases with a weighted-average interest rate of 8.7%. As of December 31, 2016, future payments under these capital leases are as follows:



2017	$	2,319
2018	1,887
2019	1,777
2020	575
2021	338
Thereafter	295
Total minimum lease payments	7,191
Less amount representing interest	(1,163		)
Future minimum lease payments	6,028
Less current portion	(1,890		)
Capital lease obligations, less current portion	$	4,138

The value of the equipment under capital lease as of December 31, 2015 and 2016 was $8,970 and $10,168, with related accumulated amortization of $2,766 and $4,073, respectively.

The Company also leases certain fueling station equipment to certain customers under sales-type leases at a weighted average effective interest rate of 12.1%. The leases are payable in varying monthly installments through September 2030.

As of December 31, 2016, future receipts under these leases are as follows:



2017	$	380
2018	251
2019	186
2020	186
2021	186
Thereafter	1,612
Total	2,801
Less amount representing interest	(1,394		)
	$	1,407

401(k) Plan	12 Months Ended
401(k) Plan	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
401(k) Plan	401(k) Plan The Company has established a savings plan ("Savings Plan") which is qualified under Section 401(k) of the Internal Revenue Code. Eligible employees may elect to make contributions to the Savings Plan through salary deferrals of up to 90% of their base pay, subject to Internal Revenue Code limitations. The Company may also make discretionary contributions to the Savings Plans that are subject to limitations. For the years ended December 31, 2014, 2015 and 2016 the Company contributed approximately $1,040, $304, and $1,527 of matching contributions to the Savings Plan, respectively.

Geographic Information

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

Geographic Information

Disclosures are required for certain information regarding operating segments, products and services, geographic areas of operation and major customers. Segment reporting is based upon the “management approach,” which assesses, how management organizes the Company’s operating segments for which separate financial information is (1) available and (2) evaluated regularly by the Chief Operating Decision Maker ("CODM") in deciding how to allocate resources and in assessing performance. The Company’s CODM is the Chief Executive Officer.

The Company operates in a single segment to sell natural gas. In making operating decisions, the CODM primarily considers consolidated financial information, accompanied by information about revenue disaggregated by geographic region. The assessment of operating results and the allocation of resources among the components of the business by the CODM are made on a project by project basis, rather than on a component-by-component basis, and are based on evaluating the economics of a mix of products and services for a customer.

The table below presents the Company's revenue, operating income (loss) and long-lived assets by geographic area. Several of the Company's functions, including marketing, engineering, and finance are performed at the corporate level. As a result, significant interdependence and overlap exists among the Company's geographic areas. Geographic revenue data reflect internal allocations and are therefore subject to certain assumptions and the Company’s methodology. Accordingly, revenue, operating income (loss), and long-lived assets shown for each geographic area may not be the amounts that would have been reported if the geographic areas were independent of one another. Revenue by geographic area is based on where services are rendered and finished goods are sold. Operating income (loss) by geographic area is based on the location of the entity selling the finished goods or providing the services. Long-lived assets by geographic area is based on the location of the assets.



	2014			2015			2016
Revenue:
United States	$	360,881		$	330,003		$	378,497
Canada	16,241			21,818			11,502
Other	51,818			32,499			12,657
Total revenue	$	428,940		$	384,320		$	402,656
Operating income (loss):
United States	$	(41,543	)	$	(33,067	)	$	(8,693	)
Canada	(3,087		)	(4,980		)	(4,212		)
Other	(9,734		)	(3,576		)	(4,732		)
Total operating income (loss)	$	(54,364	)	$	(41,623	)	$	(17,637	)
Long-lived assets:
United States	$	582,028		$	582,644		$	547,279
Canada	85,984			68,292			66,191
Other	6,854			5,693			5,646
Total long-lived assets	$	674,866		$	656,629		$	619,116

The Company's goodwill and intangible assets as of December 31, 2014, 2015 and 2016 relate to its United States operations, including the operations of Clean Energy Compression, Clean Energy Cryogenics and NG Advantage (beginning on October 14, 2014, see Note 2).

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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+ Details

Concentrations	12 Months Ended
Concentrations	Dec. 31, 2016
Risks and Uncertainties [Abstract]
Concentrations	Concentrations During the years ended December 31, 2014, 2015 and 2016, two, three and four suppliers, respectively, each accounted for 10% or more of the Company's natural gas expense related to LNG and CNG purchases. During the years ended December 31, 2014, 2015 and 2016, no single customer accounted for 10% or more of the Company's total revenue.

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company has evaluated subsequent events and determined that no subsequent events have occurred that would require recognition in the financial statements or disclosure in the notes thereto other than as listed below. On February 27, 2017, Renewables entered into an asset purchase agreement (the “APA”) with BP Products North America, Inc. (“BP”), pursuant to which Renewables agreed to sell to BP certain assets relating to its RNG production business, including Renewables' two existing RNG production facilities and its interest in the RNG Ventures (collectively, the “RNG Assets”) in exchange for $155,000 in cash and the right to receive up to an additional $25,000 in cash if certain performance criteria relating to the Assets are met (the “Asset Sale”). Subject to the satisfaction of customary closing conditions, including, among others, the receipt of required governmental and third-party consents and approvals, the Asset Sale is expected to close on or before March 31, 2017. In addition, we will collect royalties on gas purchased from BP and sold as Redeem at our stations. This royalty payment is in addition to any payment obligation of BP under the APA. At the closing of the Asset Sale, BP will pay $30,000 of the closing purchase price in cash and deliver to Renewables a promissory note for $125,000 to be paid on April 3, 2017. Following completion of the Asset Sale, the Company, through Renewables, will continue to obtain RNG from third-party producers, including RNG produced from the production facilities to be sold in the Asset Sale and sold to Renewables under a long-term supply contract, and resell such RNG through its natural gas fueling infrastructure as Redeem™, the Company’s RNG vehicle fuel. In February 2017, the Company purchased from Pickens, the 7.5% Note due July 2018 having an outstanding principal amount of $25,000 held by Pickens for a cash purchase price of $21,750.

Schedule II: Valuation and Qualifying Accounts

12 Months Ended

Dec. 31, 2016

Valuation and Qualifying Accounts [Abstract]

Schedule II: Valuation and Qualifying Accounts



	Allowances for Doubtful Trade Receivables			Allowance for Doubtful Notes Receivables
Balance as of December 31, 2013	$	832		$	2,416
Charges (benefit) to operations	387			890
Deductions	(467		)	(456		)
Balance as of December 31, 2014	752			2,850
Charges (benefit) to operations	1,514			1,142
Deductions	(371		)	(2		)
Balance as of December 31, 2015	1,895			3,990
Charges (benefit) to operations	1,107			1,617
Deductions	(1,939		)	(4,377		)
Balance as of December 31, 2016	$	1,063		$	1,230

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments necessary to state fairly the Company's financial position, results of operations, comprehensive loss and cash flows in accordance with accounting principles generally accepted in the United States of America ("US GAAP"). All intercompany accounts and transactions have been eliminated.
Reclassifications	Reclassifications Prior period Interest income of $363 and $843 for the years ended December 31, 2014 and 2015, respectively, were reclassified from Interest expense, net as a separate line item to conform to the classifications used to prepare the consolidated financial statements for the year ended December 31, 2016.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and may result in material effects on the Company's operating results and financial position. Significant estimates made in preparing the consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, fair value measurements and stock-based compensation expense.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of three months or less on the date of acquisition to be cash equivalents.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying values of the Company's financial instruments, including cash and cash equivalents, restricted cash, short-term investments, accounts and other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable, approximate their respective fair values.
Inventory	Inventory Inventory consists of raw materials and spare parts, work in process and finished goods and is stated at the lower of cost (first-in, first-out) or market. The Company writes down the carrying value of its inventory to net realizable value for estimated obsolescence or unmarketable inventory in an amount equal to the difference between the cost of inventory and its estimated realizable value based upon assumptions about future demand and market conditions, among other factors.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost. Depreciation and amortization are recognized over the estimated useful lives of the assets using the straight-line method. The estimated useful lives of depreciable assets are three to twenty years for LNG liquefaction plant assets, up to10 years for station equipment and LNG trailers, and three to seven years for all other depreciable assets. Leasehold improvements are amortized over the shorter of their estimated useful lives or related lease terms. Periodically, the Company receives grant funding to assist in the financing of natural gas fueling station construction. The Company records the grant proceeds as a reduction of the cost of the respective asset.
Long-Lived Assets	Long-Lived Assets The Company reviews long-lived assets, which include property and equipment and intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. Recoverability of long-lived assets to be held and used is measured by comparing the carrying amount of an asset to future undiscounted cash flows expected to be generated by the asset or asset group. Estimated future cash flows are determined by management based on a number of estimates, including future cash flow projections, discount rates and terminal values. In determining these estimates, management considers internally generated information and information obtained from discussions with market participants. The determination of fair value requires significant judgment by both management and outside experts engaged to assist in this process. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the fair value of the assets.
Finite-Lived Intangible Assets Amortization	Intangible assets with finite useful lives are amortized over their respective estimated useful lives using the straight-line method. The estimated useful lives of intangible assets with finite useful lives are two to 20 years for technology, one to eight years for customer relationships, one to 10 years for acquired contracts, two to 10 years for trademarks and trade names, and three years for non-compete agreements.
Goodwill	Goodwill Goodwill represents the excess of costs incurred over the fair value of the net assets of acquired businesses. The Company assesses its goodwill using a qualitative approach to determine whether it is more likely than not that the fair value of its reporting unit is less than its carrying value. The qualitative assessment includes the potential impact on a reporting unit's fair value of certain events and circumstances, including the Company's market capitalization value, macroeconomic conditions, industry and market considerations, cost factors, and other relevant entity-specific events. If it is determined, based upon the qualitative assessment, that it is more likely than not that the reporting unit's fair value is less than its carrying amount, then a two-step quantitative impairment test is performed. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review.
Revenue Recognition	Revenue Recognition The Company recognizes revenue on various products and services. The Company’s volume related revenue primarily consists of CNG, LNG and RNG fuel sales, RINs and LCFS Credits sales and O&M services. This revenue is recognized when the following four criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred and title and the risks and rewards of ownership have been transferred to the customer or services have been rendered; (iii) the price is fixed or determinable; and (iv) collectability is reasonably assured. Applying these factors, the Company typically recognizes revenue from the sale of natural gas fuel at the time the fuel is dispensed or, in the case of LNG sales agreements, delivered to the customers' storage facilities. The Company recognizes revenue from O&M service agreements as the related services are provided. The Company generates LCFS Credits when it sells RNG and conventional natural gas for use as a vehicle fuel in California and Oregon and it generates RIN Credits when it sells RNG as a vehicle fuel in the United States. The Company can sell these credits to third parties who need the credits to comply with federal and state requirements. RIN and LCFS Credits are included in volume related revenues. The Company recognizes revenue from the generation of these credits when it has an agreement in place to sell the credits at a fixed or determinable price. Compressor Sales The Company recognizes compression revenues through its subsidiary Clean Energy Compression when it sells non-lubricated natural gas fueling compressors and other equipment. Clean Energy Compression uses the percentage-of-completion method of accounting to recognize revenue because its projects are small and it has been able to demonstrate that it can reasonably estimate costs to complete. In these circumstances, revenue is recognized based on costs incurred in relation to total estimated costs to be incurred for a project. Station Construction Sales Beginning January 1, 2016, the Company began using the percentage of completion method to recognize revenue for station construction projects using the cost-to-cost method. Under this method, the Company estimates the percentage of completion of a project based on the costs incurred to date for the associated contract in comparison to the estimated total costs for such contract at completion. Historically, the Company recognized revenue on station construction projects using the completed contract method because the Company did not have a reliable means to make estimates of the percentage of the contract completed. Under the completed contract method, the construction projects were considered substantially complete at the earlier of customer acceptance of the fueling station or the time when fuel dispensing activities at the station began. The sale of compressors and related equipment continues to be recognized under the percentage of completion method as in previous periods. Effective January 1, 2016, the Company implemented a cost tracking system that provides for a detailed tracking of costs incurred on its station construction projects on a project by project basis. The Company also changed related accounting activities and processes to timely identify and monitor costs. As a result of this implementation, the Company is able to make reliable estimates as to the percentage of a project that is complete at the end of each reporting period. Station construction contracts are generally short-term, except for certain larger and more complex stations, which can take up to 24 months to complete. Management evaluates the performance of contracts on an individual contract basis. Contract price and cost estimates are reviewed periodically as work progresses and adjustments proportionate to the percentage of completion are reflected in contract revenues in the reporting period when such estimates are revised. The nature of accounting for contracts is such that refinements of estimates to account for changing conditions and new developments are continuous and characteristic of the process. Many factors that can affect contract profitability may change during the performance period of a contract, including differing site conditions, the availability of skilled contract labor, the performance of major suppliers and subcontractors, and unexpected changes in material costs. Changes to these factors may result in revisions to costs and income, which are recognized in the period in which the revisions become known. Provisions for estimated losses on uncompleted contracts are made in the period in which such losses become known. During the year ended December 31, 2016, there were no significant losses on open contracts. The Company considers unapproved change orders to be contract variations for which the customer has approved the change of scope but an agreement has not been reached as to an associated price change. Change orders that are unapproved as to both price and scope are evaluated as claims. Claims have historically been insignificant. There were no significant unapproved change orders, claims, contract penalties, settlements or changes in contract estimates during the year ended December 31, 2016. As a result of using the percentage of completion method to recognize revenues, revenue and operating income from station construction sales during the year ended December 31, 2016 were higher by $18,683 and $1,666, respectively, than would have been recognized during the period under the completed contract method. Additionally, net loss per diluted share was $0.01 lower than what would have been reported using the completed contract method. In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction of and sale of a station, providing O&M services to the station, and sale of fuel to the customer. The Company evaluates the separability of revenues based on Financial Accounting Standards Board ("FASB") authoritative guidance, which provides a framework for establishing whether or not a particular arrangement with a customer has one or more revenue elements, and allows the Company to use a combination of internal and external objective and reliable evidence to develop management's best estimate of the fair value of the contract elements. If the arrangement contains a lease, the Company uses the existing evidence of fair value to separate the lease from the other elements in the arrangement. The arrangement's consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the estimated relative selling price of each deliverable, which is determined based on the historical data derived from the Company's stand-alone projects. The revenue allocated to the construction of the station is recognized using the percentage of completion method. The revenue allocated to the O&M services is recognized ratably over the term of the arrangement and sale of fuel is recognized as the fuel is delivered. See the discussion under “Alternative Fuels Excise Tax Credit” below for further information. Other The Company collects and remits taxes assessed by various governmental authorities that are imposed on and concurrent with revenue-producing transactions between the Company and our customers. These taxes may include, but are not limited to, fuel, sales and value-added taxes. The Company reports the collection of these taxes on a net basis.
Alternative Fuels Excise Tax Credit	Alternative Fuels Excise Tax Credit Under separate pieces of U.S. federal legislation from October 1, 2006 through December 31, 2014, the Company was eligible to receive a federal alternative fuels tax credit (“VETC”) of $0.50 per gasoline gallon equivalent of CNG and $0.50 per liquid gallon of LNG that it sold as vehicle fuel. In December 2015, another alternative fuels tax credit, which the Company also refers to as VETC, was made available for the period from January 1, 2015 through December 31, 2016. This new credit was the same as the credit it replaced, except that the alternative fuels tax credit for LNG sold as a vehicle fuel in 2016 was based on the diesel gallon equivalent of LNG sold rather than the liquid gallon of LNG sold. Based on the service relationship with its customers, either the Company or its customers claims the credit. The Company records its VETC credits, if any, as revenue in its consolidated statements of operations as the credits are fully payable and do not need to offset income tax liabilities to be received.
LNG Transportation Costs	LNG Transportation Costs The Company records the costs incurred to transport LNG to its customers in the line item product cost of sales in the accompanying consolidated statements of operations.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company recognizes compensation expense for all stock‑based payment arrangements, net of an estimated forfeiture rate, over the requisite service period of the award. For stock options, the Company determines the grant date fair value using the Black‑Scholes option pricing model, which requires the input of certain assumptions, including the expected life of the stock‑based payment awards, stock price volatility and risk‑free interest rates. For restricted stock units, the Company determines the grant date fair value based on the closing market price of its common stock on the date of grant.
Income Taxes	Income Taxes Income taxes are computed using the asset and liability method. Under this method, deferred income taxes are recognized by applying enacted statutory tax rates applicable to future years to differences between the tax bases and financial carrying amounts of existing assets and liabilities. The impact on deferred taxes of changes in tax rates and laws, if any, are applied to the years during which temporary differences are expected to be settled and are reflected in the consolidated financial statements in the period of enactment. Valuation allowances are established when management determines it is more likely than not that deferred tax assets will not be realized. When evaluating the need for a valuation analysis, we use estimates involving a high degree of judgment including projected future US GAAP income and the amounts and estimated timing of the reversal of any deferred tax assets and liabilities. The Company has a recognition threshold and a measurement attribute for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities based on the technical merits of the position. The amount recognized is measured as the largest amount of benefit that has a greater than 50 percent likelihood of being realized upon ultimate settlement. The Company recognizes potential accrued interest and penalties related to unrecognized tax benefit in income tax expense. The Company operates within multiple domestic and foreign taxing jurisdictions and is subject to audit in these jurisdictions. These audits can involve complex issues, which may require an extended period of time to resolve. Although the Company believes that adequate consideration has been given to these issues, it is possible that the ultimate resolution of these issues could be significantly different than originally estimated.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period. Diluted net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period and potentially dilutive securities outstanding during the period, and therefore reflects the dilution from common shares that may be issued upon exercise or conversion of these potentially dilutive securities, such as stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock awards and warrants is computed under the treasury stock method. The dilutive effect of convertible notes and restricted stock units is computed under the if-converted method. Potentially dilutive securities are excluded from the computations of diluted net loss per share if their effect would be antidilutive.
Foreign Currency Translation	Foreign Currency Translation The Company uses the local currency as the functional currency of its foreign subsidiaries. Accordingly, all assets and liabilities outside the United States are translated into U.S. dollars at the rate of exchange in effect at the balance sheet date. Revenue and expense items are translated at the weighted-average exchange rates prevailing during the period. Foreign currency translation adjustments are recorded as accumulated other comprehensive income (loss) in stockholders' equity.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in equity (net assets) of a business enterprise during the period from transactions and other events and circumstances from non-owner sources.
Concentration of Credit Risk	Concentration of Credit Risk Credit is extended to all customers based on financial condition, and collateral is generally not required. Concentrations of credit risk with respect to trade receivables are limited because of the large number of customers comprising the Company's customer base and dispersion across many different industries and geographies. Certain international customers, however, have historically been slower to pay on trade receivables. Accordingly, the Company continually monitors collections and payments from its customers and maintains a provision for estimated credit losses based upon its historical experience and any specific customer collection issues that it has identified. In addition, through Export Development Canada, Clean Energy Compression maintains accounts receivable insurance on a substantial portion of its foreign trade receivables, which covers up to 90% of the related outstanding balance. Although credit losses have historically been within the Company's expectations and the provisions established, the Company cannot guarantee that it will continue to experience the same credit loss rates that it has in the past. The Company places its cash and cash equivalents with high credit quality financial institutions. At times, such investments may be in excess of the Federal Deposit Insurance Corporation ("FDIC"), Canadian Deposit Insurance Corporation ("CDIC"), and other foreign insurance limits. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash deposits.
Recently Adopted Accounting Changes and Recently Issued and Adopted Accounting Standards	Recently Adopted Accounting Changes and Recently Issued and Adopted Accounting Standards In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new ASU, goodwill impairment testing will be performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value. The new standard is effective for annual and interim goodwill impairment tests in fiscal years beginning after December 15, 2019, which for the Company is the first quarter of 2020 and should be applied on a prospective basis. Early adoption is permitted for annual or interim goodwill impairment testing performed after January 1, 2017. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The new standard provides guidance to entities to assist with evaluating when a set of transferred assets and activities (collectively, the "set") is a business and provides a criteria to determine when a set is not a business. Under the new guidance, when substantially all of the fair value of gross assets acquired (or disposed of) is concentrated in a single identifiable asset, or group of similar assets, the assets acquired would not represent a business. Also, to be considered a business, an acquisition would have to include an input and a substantive process that together significantly contribute to the ability to produce outputs. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017, which for the Company is the first quarter of 2018 and should be applied on a prospective basis to any transactions occurring within the period of adoption. Early adoption is permitted for interim or annual periods in which the financial statements have not been issued. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures. In December 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The new standard requires restricted cash and restricted cash equivalents to be included as components of total cash and cash equivalents as presented on the statement of cash flows. This pronouncement is effective for reporting periods beginning after December 15, 2017, which for the Company is the first quarter of 2018. Early adoption is allowed in an interim or annual reporting period. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures. In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. Under the new standard, the selling (transferring) entity is required to recognize a current tax expense or benefit upon transfer of the asset. Similarly, the purchasing (receiving) entity is required to recognize a deferred tax asset or liability, as well as the related deferred tax benefit or expense, upon purchase or receipt of the asset. This pronouncement is effective for reporting periods beginning after December 15, 2017, which for the Company is the first quarter of 2018, with early adoption permitted. The Company will early adopt the standard as of January 1, 2017. The impact of adoption will not have a material impact to the Company's financial statements. In September 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Payments. The new standard provides clarification as to the classification of certain transactions as operating, investing or financing activities. This pronouncement is effective for reporting periods beginning after December 15, 2017, with early adoption permitted. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard amends the impairment model to utilize an expected loss methodology in place of the currently used incurred loss methodology, which will result in the more timely recognition of losses. This pronouncement is effective for reporting periods beginning after December 15, 2019, which for the Company is the first quarter of 2020. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In March 2016, the FASB issued ASU No. 2016-09, Compensation – Stock Compensation (Topic 718) ): Improvements to Employee Share-Based Payments Accounting. The new standard was issued to simplify the accounting for share-based payment transactions, including income tax consequences, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. The Company will adopt the standard in the first quarter of fiscal 2017, prospectively. In connection with the adoption, the Company will elect to recognize forfeitures when they occur. Previously, the Company estimated a forfeiture rate in accordance with prior guidance. This election will be implemented under the modified retrospective approach, resulting in an immaterial decrease to retained earnings representing the cumulative additional compensation expense that would have been amortized through the date of adoption had this accounting policy election been in place. In February 2016, the FASB issued ASU No. 2016-02, Leases. The new standard requires most leases to be recognized on the balance sheet which will increase reported assets and liabilities. Lessor accounting remains substantially similar to current guidance. The new standard is effective for annual and interim periods in fiscal years beginning after December 15, 2018, which for the Company is the first quarter of 2019, and mandates a modified retrospective transition method. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The new standard requires equity investments to be measured at fair value with changes in fair value recognized in net income, simplifies the impairment assessment of equity investments without readily determinable fair values, eliminates the requirement to disclose the methods and significant assumptions used to estimate fair value, requires use of the exit price notion when measuring fair value, requires separate presentation in certain financial statements, and requires an evaluation of the need for a valuation allowance on a deferred tax asset related to available-for-sale securities. The new standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In May 2014, the FASB issued an accounting standard update related to revenue from contracts with customers, which, along with amendments issued in 2015 and 2016, will provide a single, comprehensive revenue recognition model for all contracts with customers. The underlying principle is to recognize revenue when promised goods or services are transferred to customers in amounts that reflect the consideration that is expected to be received for those goods or services. The new standard also requires entities to enhance disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. This accounting standard update, as amended, will be effective for annual reporting periods after December 15, 2017, which for the Company is the first quarter of fiscal 2018. The new revenue standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized in retained earnings as of the date of adoption ("modified retrospective basis"). The Company expects to adopt this accounting standard update on a modified retrospective basis and is currently evaluating the impact of this accounting standard update on its consolidated financial statements and related disclosures.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of inventories

Inventories consisted of the following as of December 31, 2015 and 2016:



	2015		2016
Raw materials and spare parts	$	25,113	$	24,843
Work in process	973		845
Finished goods	3,203		3,856
Total inventory	$	29,289	$	29,544

Schedule of the Company's intangible assets

The Company's intangible assets as of December 31, 2015 and 2016 were as follows:



	2015			2016
Technology	$	54,400		$	54,400
Customer relationships	16,576			16,576
Acquired contracts	3,694			4,384
Trademark and trade names	8,200			8,200
Non-compete agreements	2,060			2,060
Total intangible assets	84,930			85,620
Less accumulated amortization	(30,442		)	(37,591		)
Foreign currency rate change	(11,844		)	(9,329		)
Net intangible assets	$	42,644		$	38,700

Summary of revenue by group of similar products

The table below and the following discussion describe the Company’s revenue by group of similar products.



	Year Ended December 31,
(in thousands)	2014		2015		2016
Volume Related	$	247,899	$	260,629	$	283,814
Compressor Sales	84,775		54,497		27,262
Station Construction Sales	67,392		37,830		64,942
VETC	28,359		30,986		26,638
Other	515		378		—
	$	428,940	$	384,320	$	402,656

Schedule of potentially dilutive securities that have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive

The following potentially dilutive securities have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive:



	2014	2015	2016
Stock options	11,486,301	11,487,938	11,467,796
Warrants	6,130,682	2,130,682	—
Convertibles notes	35,185,979	35,185,979	16,573,799
Restricted stock units	2,591,752	3,419,776	2,072,304

Acquisitions and Divestitures (Tables)

12 Months Ended

Dec. 31, 2016

Business Combinations [Abstract]

Summary of allocation of the aggregate purchase price to the estimated fair value of the assets acquired and liabilities assumed as of the acquisition date

The following table summarizes the allocation of the aggregate purchase price to the fair value of the assets acquired and liabilities assumed:



Current assets	$	40,558
Property, plant and equipment	20,862
Other long-term assets	5,115
Identifiable intangible assets	5,600
Goodwill	21,070
Total assets acquired	93,205
Current liabilities assumed	(9,165		)
Long-term debt including capital leases assumed, excluding current installments	(17,604		)
Other liabilities	(711		)
Noncontrolling interest	(28,075		)
Total purchase price	$	37,650

Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2016

Cash and Cash Equivalents [Abstract]

Schedule of components of restricted cash

Restricted cash as of December 31, 2015 and 2016 consisted of the following:



	December 31, 2015		December 31, 2016
Short-term restricted cash:
Standby letters of credit	$	1,631	$	1,753
Canton Bonds (see Note 10)	2,609		3,665
Held in escrow	—		1,578
Total short-term restricted cash	$	4,240	$	6,996

Investments (Tables)

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Summary of short-term investments

Short-term investments as of December 31, 2015 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	16,797	$	(7	)	$	16,790
Zero coupon bonds	500		(1		)	499
Corporate bonds	37,181		(77		)	37,104
Certificate of deposits	48,551		—			48,551
Total short-term investments	$	103,029	$	(85	)	$	102,944

Short-term investments as of December 31, 2016 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	8,791	$	(4	)	$	8,787
Corporate bonds	21,517		(7		)	21,510
Certificate of deposits	43,421		—			43,421
Total short-term investments	$	73,729	$	(11	)	$	73,718

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Schedule of information by level for assets and liabilities that are measured at fair value on a recurring basis

The following tables provide information by level for assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2015 and December 31, 2016, respectively:



Description	Balance at December 31, 2015		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	16,790	$	—	$	16,790	$	—
Zero coupon bonds	499		—		499		—
Corporate bonds	37,104		—		37,104		—
Certificate of deposits	48,551		—		48,551		—
Liabilities:
Warrants(2)	632		—		—		632



Description	Balance at December 31, 2016		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	8,787	$	—	$	8,787	$	—
Corporate bonds	21,510		—		21,510		—
Certificate of deposits	43,421		—		43,421		—
Liabilities:
Warrants(2)	581		—		—		581

_______________________________________________________________________________


(1)	Included in short-term investments in the consolidated balance sheets. See Note 4 for further information.


(2)	Included in accrued liabilities and other long-term liabilities in the consolidated balance sheets.

Reconciliation of the beginning and ending balances of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)



Liabilities: Warrants	2015			2016
Beginning Balance	$	2,046		$	632
Gain included in earnings	(1,414		)	(51		)
Ending Balance	$	632		$	581

Other Receivables (Tables)

12 Months Ended

Dec. 31, 2016

Receivables [Abstract]

Schedule of other receivables

Other receivables as of December 31, 2015 and 2016 consisted of the following:



	2015		2016
Loans to customers to finance vehicle purchases	$	10,531	$	7,416
Accrued customer billings	7,106		4,308
Fuel tax credits	40,730		6,358
Other	2,300		3,852
Total other receivables	$	60,667	$	21,934

Land, Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2016

Property, Plant and Equipment [Abstract]

Summary of land, property and equipment

Land, property and equipment as of December 31, 2015 and 2016 consisted of the following:



	2015			2016
Land	$	2,858		$	2,858
LNG liquefaction plants	94,634			94,634
RNG plants	46,397			47,545
Station equipment	316,258			341,605
Trailers	50,414			54,985
Other equipment	83,687			93,118
Construction in progress	139,586			117,662
	733,834			752,407
Less accumulated depreciation	(217,510		)	(268,484		)
Total land, property and equipment, net	$	516,324		$	483,923

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2016

Payables and Accruals [Abstract]

Schedule of accrued liabilities

Accrued liabilities as of December 31, 2015 and 2016 consisted of the following:



	2015		2016
Accrued alternative fuels incentives (1)	15,651		9,840
Accrued employee benefits	3,042		4,317
Accrued interest	3,718		1,849
Accrued gas and equipment purchases	14,133		11,657
Accrued property and other taxes	5,344		4,572
Salaries and wages	9,537		12,293
Other	7,657		8,073
Total accrued liabilities	$	59,082	$	52,601

(1) Includes VETC and tradable RINs and LCFS Credits (discussed in Note 1) payable to third parties.

Debt (Tables)

12 Months Ended

Dec. 31, 2016

Debt Disclosure [Abstract]

Schedule of long-term debt

Debt and capital lease obligations as of December 31, 2015 and 2016 consisted of the following and are further discussed below:



				December 31, 2015
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes (1)	$	150,000		399			$	149,601
SLG Notes	145,000			38			$	144,962
5.25% Notes	250,000			3,985			$	246,015
Canton Bonds	10,910			514			$	10,396
Capital lease obligations	6,448			—			$	6,448
Other debt	10,056			328			$	9,728
Total debt and capital lease obligations	572,414			5,264			567,150
Less amounts due within one year	(150,129		)	(273		)	(149,856		)
Total long-term debt and capital lease obligations	$	422,285		$	4,991		$	417,294



				December 31, 2016
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes (1)	150,000			274			$	149,726
5.25% Notes	110,450			1,088			$	109,362
PlainsCapital Bank Credit Facility	23,500			—			23,500
Canton Bonds	9,520			373			$	9,147
Capital lease obligations	6,028			—			$	6,028
Other debt	14,850			237			$	14,613
Total debt and capital lease obligations	314,348			1,972			312,376
Less amounts due within one year	(6,126		)	(183		)	(5,943		)
Total long-term debt and capital lease obligations	$	308,222		$	1,789		$	306,433

Summary of aggregate maturities of long term debt and capital lease obligations

The following is a summary of the aggregate maturities of debt and capital lease obligations for each of the years ending December 31, as of December 31, 2016:



	2017		2018		2019		2020		2021		Thereafter
7.5% Notes (1)	—		50,000		50,000		50,000		—		—
5.25% Notes	—		110,450		—		—		—		—
PlainsCapital Bank Credit Facility	—		23,500		—		—		—		—
Canton Bonds	1,420		1,460		1,555		1,665		1,695		1,725
Capital lease obligations	1,890		1,569		1,568		462		265		274
Other debt	2,816		2,667		2,370		2,507		2,036		2,454
Total	$	6,126	$	189,646	$	55,493	$	54,634	$	3,996	$	4,453

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2016

Stockholders' Equity Note [Abstract]

Summary of At-The-Market program activity

The following table summarizes the activity under the ATM Program for the periods presented:



	Year ended December 31,		Year ended December 31,
(in 000s, except for per-share amounts)	2015		2016
Gross proceeds	$	6,943	$	103,591
Fees and issuance costs	$	493	$	2,612
Net proceeds	$	6,450	$	100,979
Shares issued	1,561,902		31,064,434

Summary of compensation expense and related income tax benefit related to the stock-based compensation expense recognized



	Years Ended December 31,
	2014		2015		2016
Stock-based compensation expense, net of $0 tax in 2014, 2015 and 2016	$	11,514	$	10,779	$	8,092

Summary of stock option activity

The following table summarizes Renewables' unit option activity:



	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding as of December 31, 2013	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding as of December 31, 2014	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	(7,000	)	40.80
Options Outstanding as of December 31, 2015	108,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding and Exercisable as of December 31, 2016	108,000		$	40.80	6.72	$	94.62

The following table summarizes the Company's stock option activity:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding as of December 31, 2013	11,526,998		$	11.79
Granted	957,000		10.23
Exercised	(468,279	)	3.78
Forfeited or Expired	(529,418	)	13.40
Options Outstanding as of December 31, 2014	11,486,301		$	11.91
Granted	1,415,200		5.39
Exercised	(608,279	)	2.96
Forfeited or Expired	(805,284	)	14.04
Options Outstanding as of December 31, 2015	11,487,938		$	11.44
Granted	284,750		3.63
Exercised	—		—
Forfeited or Expired	(304,892	)	11.30
Options Outstanding as of December 31, 2016	11,467,796		$	11.25	3.99	—
Options Exercisable as of December 31, 2016	10,176,324		$	11.96	3.41	—
Options Vested and Expected to Vest as of December 31, 2016	11,351,637		$	11.30	3.88	—

Schedule of assumptions used to estimate the fair value of each award using the Black-Scholes option pricing model

The fair value of each stock option granted was estimated as of the date of grant using the Black-Scholes option pricing model and using the following assumptions:



	Years Ended December 31,
	2014	2015	2016
Dividend yield	0.0%	0.0%	0.0%
Expected volatility	52.3% to 67.0%	59.2% to 72.0%	61.1% to 70.8%
Risk-free interest rate	1.1% to 1.8%	1.7% to 1.8%	1.2% to 2.0%
Expected life in years	6.0	6.0	6.0

Summary of the Company's RSU activity

The following table summarizes the Company's Service-Based RSU activity:



	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
RSU Outstanding as of December 31, 2013	45,836		$	13.09
Granted	792,500		5.54
Vested	(15,584	)	13.09
Forfeited or expired	—		—
RSU Outstanding as of December 31, 2014	822,752		$	5.82
Granted	1,167,750		5.38
Vested	(283,726	)	5.94
Forfeited or expired	(56,000	)	5.57
RSU Outstanding as of December 31, 2015	1,650,776		$	5.50
Granted	850,125		3.63
Vested	(726,687	)	5.53
Forfeited or expired	(130,910	)	4.91
RSU Outstanding and Unvested as of December 31, 2016	1,643,304		$	4.56	1.19
RSU Expected to Vest as of December 31, 2016	1,545,222		$	4.56	0.99

The following table summarizes the Company's Market-Based RSU activity:



	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
RSU Outstanding as of December 31, 2013	1,545,000		$	11.42
Granted	489,500		8.26
Vested	—		—
Forfeited or Expired	(265,500	)	10.62
RSU Outstanding as of December 31, 2014	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding as of December 31, 2015	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	(1,340,000	)	11.44
RSU Outstanding and Unvested as of December 31, 2016	429,000		$	8.26	1.09
RSU Expected to Vest as of December 31, 2016	—		—		0.00

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Schedule of components of income (loss) before income taxes

The components of income (loss) before income taxes for the years ended December 31, 2014, 2015 and 2016 are as follows:



	2014			2015			2016
U.S.	$	(64,913	)	$	(111,437	)	$	7,150
Foreign	(24,871		)	(22,407		)	(19,535		)
Total loss before income taxes	$	(89,784	)	$	(133,844	)	$	(12,385	)

Schedule of provision for income taxes

The provision for income taxes consists of the following:



	2014			2015		2016
Current:
Federal	$	190		$	9	$	(226	)
State	238			248		93
Foreign	1,017			912		567
Total current	1,445			1,169		434
Deferred:
Federal	29			337		478
State	(10		)	71		75
Foreign	(389		)	37		352
Total deferred	(370		)	445		905
Total	$	1,075		$	1,614	$	1,339

Schedule of reconciliation of federal income tax rate to the actual effective tax rate

Income tax expense for the years ended December 31, 2014, 2015 and 2016 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following:



	2014			2015			2016
Computed expected tax (benefit)	$	(30,415	)	$	(46,846	)	$	(4,335	)
Nondeductible expenses	10,690			24,998			5,971
Tax rate differential on foreign earnings	5,733			3,701			720
Tax credits	(8,286		)	(9,988		)	(9,331		)
Other	(1,121		)	(372		)	833
Change in valuation allowance	24,474			30,121			7,481
Total tax expense	$	1,075		$	1,614		$	1,339

Schedule of tax effect of temporary differences that give rise to deferred tax assets and liabilities

The tax effect of temporary differences that give rise to deferred tax assets and liabilities as of December 31, 2015 and 2016 are as follows:



	2015			2016
Deferred tax assets:
Accrued expenses	$	4,512		$	4,566
Sales-type leases	154			55
Alternative minimum tax and general business credits	5,780			6,137
Stock option expense	23,113			26,154
Other	2,283			1,168
Loss carryforwards	174,157			181,884
Total deferred tax assets	209,999			219,964
Less valuation allowance	(189,203		)	(195,968		)
Net deferred tax assets	20,796			23,996
Deferred tax liabilities:
Depreciation and amortization	(17,398		)	(19,364		)
Goodwill	(4,600		)	(5,599		)
Partnership income	(293		)	(1,432		)
Total deferred tax liabilities	(22,291		)	(26,395		)
Net deferred tax liabilities	$	(1,495	)	$	(2,399	)

Schedule of reconciliation of the total amounts of unrecognized tax benefits

The following is a tabular reconciliation of the total amounts of unrecognized tax benefits for the years ended December 31, 2014, 2015 and 2016:



Unrecognized tax benefit—December 31, 2014	$	21,974
Gross increases—tax positions in current year	5,523
Gross increases—tax positions in prior years	—
Unrecognized tax benefit—December 31, 2015	27,497
Gross increases—tax positions in current year	4,556
Gross increases—tax positions in prior years	17,549
Unrecognized tax benefit—December 31, 2016	$	49,602

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2016

Commitments and Contingencies Disclosure [Abstract]

Schedule of future minimum lease obligations for all noncancelable operating leases

The following schedule represents the future minimum lease obligations for all noncancelable operating leases as of December 31, 2016:



Fiscal year:
2017	$	8,055
2018	6,685
2019	6,287
2020	5,575
2021	4,195
Thereafter	17,801
Total future minimum lease payments	$	48,598

Capitalized Lease Obligation and Receivables (Tables)

12 Months Ended

Dec. 31, 2016

Leases [Abstract]

Schedule of future payments under the capital leases

As of December 31, 2016, future payments under these capital leases are as follows:



2017	$	2,319
2018	1,887
2019	1,777
2020	575
2021	338
Thereafter	295
Total minimum lease payments	7,191
Less amount representing interest	(1,163		)
Future minimum lease payments	6,028
Less current portion	(1,890		)
Capital lease obligations, less current portion	$	4,138

Schedule of future receipts under the leases

As of December 31, 2016, future receipts under these leases are as follows:



2017	$	380
2018	251
2019	186
2020	186
2021	186
Thereafter	1,612
Total	2,801
Less amount representing interest	(1,394		)
	$	1,407

Geographic Information (Tables)

12 Months Ended

Dec. 31, 2016

Segment Reporting [Abstract]

Schedule of revenue, operating income (loss), and long-lived assets shown for each geographic area



	2014			2015			2016
Revenue:
United States	$	360,881		$	330,003		$	378,497
Canada	16,241			21,818			11,502
Other	51,818			32,499			12,657
Total revenue	$	428,940		$	384,320		$	402,656
Operating income (loss):
United States	$	(41,543	)	$	(33,067	)	$	(8,693	)
Canada	(3,087		)	(4,980		)	(4,212		)
Other	(9,734		)	(3,576		)	(4,732		)
Total operating income (loss)	$	(54,364	)	$	(41,623	)	$	(17,637	)
Long-lived assets:
United States	$	582,028		$	582,644		$	547,279
Canada	85,984			68,292			66,191
Other	6,854			5,693			5,646
Total long-lived assets	$	674,866		$	656,629		$	619,116

Summary of Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income	$ 827
Unamortized debt issuance costs reclassified	$ 1,972	$ 5,264
Prepaid expenses and other current assets \| Accounting Standards Update 2015-03
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified		(273)
Current portion of long-term debt and capital lease obligations \| Accounting Standards Update 2015-03
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified		273
Notes receivable and other long-term assets \| Accounting Standards Update 2015-03
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified		(4,991)
Long term debt and capital lease obligations \| Accounting Standards Update 2015-03
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified		4,991
Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income		$ (843)	$ (363)

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Inventories
Raw materials and spare parts	$ 24,843	$ 25,113
Work in process	845	973
Finished goods	3,856	3,203
Total inventory	29,544	29,289
Property and Equipment
Proceeds from grants	3,295	4,292	$ 959
Advertising expense	15	44	439
Foreign exchange transactions gains (losses)	$ 132	$ 975	$ (3,188)
LNG liquefaction plants \| Minimum
Property and Equipment
Estimated useful lives	3 years
LNG liquefaction plants \| Maximum
Property and Equipment
Estimated useful lives	20 years
Station equipment
Property and Equipment
Estimated useful lives	10 years
Trailers
Property and Equipment
Estimated useful lives	10 years
Other equipment \| Minimum
Property and Equipment
Estimated useful lives	3 years
Other equipment \| Maximum
Property and Equipment
Estimated useful lives	7 years

Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($)			3 Months Ended	12 Months Ended
	Dec. 29, 2014	Oct. 14, 2014	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill and intangible assets
Long-lived intangible impairment				$ 0	$ 0	$ 4,772,000
Goodwill and intangible asset impairment				0
Total intangible assets				85,620,000	84,930,000
Less accumulated amortization				(37,591,000)	(30,442,000)
Foreign currency rate change				(9,329,000)	(11,844,000)
Net intangible assets				38,700,000	42,644,000
Amortization expense				5,794,000	5,539,000	$ 7,024,000
Estimated amortization expense
2017				6,956,000
2018				5,913,000
2019				4,532,000
2020				3,835,000
2021				3,184,000
Thereafter				14,280,000
Foreign currency translation gains (losses) included in goodwill balances				1,051,000	(6,578,000)
NG Advantage
Goodwill and intangible assets
Identifiable intangible assets, customer relationships and trade names		$ 5,600,000
Estimated amortization expense
Increase in goodwill on acquisition		$ 21,070,000
IMW
Goodwill and intangible assets
Long-lived intangible impairment			$ 4,772,000
Goodwill and intangible asset impairment					0
Dallas Clean Energy, LLC
Estimated amortization expense
Reduction in goodwill on sale	$ 7,205,000
Minimum \| NG Advantage
Goodwill and intangible assets
Estimated useful lives		4 years
Maximum \| NG Advantage
Goodwill and intangible assets
Estimated useful lives		7 years
Technology
Goodwill and intangible assets
Total intangible assets				$ 54,400,000	54,400,000
Technology \| Minimum
Goodwill and intangible assets
Estimated useful lives				2 years
Technology \| Maximum
Goodwill and intangible assets
Estimated useful lives				20 years
Customer relationships
Goodwill and intangible assets
Total intangible assets				$ 16,576,000	16,576,000
Customer relationships \| Minimum
Goodwill and intangible assets
Estimated useful lives				1 year
Customer relationships \| Maximum
Goodwill and intangible assets
Estimated useful lives				8 years
Acquired contracts
Goodwill and intangible assets
Total intangible assets				$ 4,384,000	3,694,000
Acquired contracts \| Minimum
Goodwill and intangible assets
Estimated useful lives				1 year
Acquired contracts \| Maximum
Goodwill and intangible assets
Estimated useful lives				10 years
Trademarks and trade names
Goodwill and intangible assets
Total intangible assets				$ 8,200,000	8,200,000
Trademarks and trade names \| Minimum
Goodwill and intangible assets
Estimated useful lives				2 years
Trademarks and trade names \| Maximum
Goodwill and intangible assets
Estimated useful lives				10 years
Non-compete agreements
Goodwill and intangible assets
Estimated useful lives				3 years
Total intangible assets				$ 2,060,000	$ 2,060,000
Customer relationships and tradenames \| NG Advantage
Goodwill and intangible assets
Identifiable intangible assets, customer relationships and trade names		$ 5,600,000

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 402,656	$ 384,320	$ 428,940
Percentage-of-Completion Method
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Effect of change on net revenue	18,683
Effect of change on operating income	$ 1,666
Effect of change on diluted income per share (in dollars per share)	$ 0.01
Volume Related
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 283,814	260,629	247,899
Compressor Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	27,262	54,497	84,775
Station Construction Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	64,942	37,830	67,392
VETC
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	26,638	30,986	28,359
Other
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 0	$ 378	$ 515

Summary of Significant Accounting Policies - Alternative Fuels Excise Tax Credit (Details) $ in Thousands	12 Months Ended			99 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / gallon	Dec. 31, 2014 USD ($)	Dec. 31, 2014 $ / gallon
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent)		0.50		0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon)				0.50
VETC credits recognized as revenue \| $	$ 26,638	$ 30,986	$ 28,359

Summary of Significant Accounting Policies - Net Loss Per Share (Details) - shares				12 Months Ended					16 Months Ended
	Oct. 04, 2016	Dec. 31, 2014	Sep. 11, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Net Loss Per Share
Number of shares that will not vest and will not become exercisable (in shares)					4,000,000				4,000,000
Common stock, issued (in shares)				145,538,063	92,382,717				92,382,717
GE Credit Agreement \| GE Warrant
Net Loss Per Share
Number of shares exercisable in the period (in shares)		500,000						500,000
Common stock, issued (in shares)	997,740
Common stock \| GE Credit Agreement \| GE Warrant
Net Loss Per Share
Anti-dilutive securities (in shares)			4,000,000						4,000,000
Number of securities called by warrants (in shares)	1,000,000
Number of shares that will not vest and will not become exercisable (in shares)					4,000,000				4,000,000
Stock Options
Net Loss Per Share
Anti-dilutive securities (in shares)				11,467,796	11,487,938	11,486,301
Series I Warrant Liabilities
Net Loss Per Share
Anti-dilutive securities (in shares)				0	2,130,682	6,130,682
Common stock related to the GE Warrant included in the basic and dilutive net loss per share calculation (in shares)							5,000,000
Convertible Notes
Net Loss Per Share
Anti-dilutive securities (in shares)				16,573,799	35,185,979	35,185,979
Restricted Stock Units
Net Loss Per Share
Anti-dilutive securities (in shares)				2,072,304	3,419,776	2,591,752

Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Concentration of Credit Risk
Amount in excess of FDIC insurance limits	$ 34,439	$ 40,691
IMW
Concentration of Credit Risk
Maximum percentage of foreign trade receivables covered by accounts receivable insurance (up to)	90.00%

Acquisitions and Divestitures - NG Advantage (Details) $ in Thousands		12 Months Ended
	Oct. 14, 2014 USD ($) payment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Allocation of aggregate purchase price
Goodwill		$ 93,018	$ 91,967
Income (loss) from noncontrolling interest		(1,571)	(1,216)	$ (1,200)
Noncontrolling interest in subsidiary		24,822	26,393
NG Advantage
Allocation of aggregate purchase price
Current assets	$ 40,558
Property, plant and equipment	20,862
Other long-term assets	5,115
Identifiable intangible assets	5,600
Goodwill	21,070
Total assets acquired	93,205
Current liabilities assumed	(9,165)
Long-term debt including capital leases assumed, excluding current installments	(17,604)
Other liabilities	(711)
Noncontrolling interest	(28,075)
Total purchase price	37,650
Assumed debt related to purchases of capital assets and working capital needs	20,439
Debt payment	$ 10,361
Income (loss) from noncontrolling interest		(1,571)	(1,216)	$ (1,200)
Noncontrolling interest in subsidiary		24,822	$ 26,393
Payments to acquire productive assets		$ 1,550
NG Advantage \| Minimum
Allocation of aggregate purchase price
Identifiable intangible assets, estimated useful lives	4 years
NG Advantage \| Maximum
Allocation of aggregate purchase price
Identifiable intangible assets, estimated useful lives	7 years
NG Advantage \| Customer relationships and tradenames
Allocation of aggregate purchase price
Identifiable intangible assets	$ 5,600
NG Advantage \| Common unit purchase agreement
Acquisitions and Divestitures
Total consideration	$ 37,650
Ownership interest acquired	53.30%
Consideration paid in cash	$ 19,000
Consideration liability	$ 18,650
Number of installments \| payment	2
NG Advantage \| Common unit purchase agreement \| Due by January 2015
Acquisitions and Divestitures
Consideration liability	$ 3,000
NG Advantage \| Common unit purchase agreement \| Due by April 2015
Acquisitions and Divestitures
Consideration liability	$ 15,650

Acquisitions and Divestitures - DCE and DCEMB (Details) - USD ($) $ in Thousands			1 Months Ended			4 Months Ended	12 Months Ended
	Dec. 29, 2014	Sep. 04, 2014	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Divestitures
Gain from sale of subsidiary							$ 0	$ 937	$ 11,998
Goodwill							$ 93,018	91,967
Dallas Clean Energy, LLC \| Cambrian
Divestitures
Cash consideration received on sale of subsidiary			$ 1,118	$ 40,588	$ 6,992
Dallas Clean Energy, LLC \| Cambrian \| Mavrix
Divestitures
Percentage of ownership sold	51.00%	19.00%
Ownership interest		70.00%
Gain from sale of subsidiary		$ 12,935				$ 11,998		$ 937

Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Restricted Cash
Short-term restricted cash	$ 6,996	$ 4,240
Canton Bonds
Restricted Cash
Short-term restricted cash	3,665	2,609
Held in Escrow
Restricted Cash
Short-term restricted cash	1,578	0
Standby letters of credit
Restricted Cash
Short-term restricted cash	$ 1,753	$ 1,631

Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Short-term investments
Amortized Cost	$ 73,729	$ 103,029
Gross Unrealized Losses	(11)	(85)
Estimated Fair Value	73,718	102,944
Municipal bonds and notes
Short-term investments
Amortized Cost	8,791	16,797
Gross Unrealized Losses	(4)	(7)
Estimated Fair Value	8,787	16,790
Zero coupon bonds
Short-term investments
Amortized Cost		500
Gross Unrealized Losses		(1)
Estimated Fair Value		499
Corporate bonds
Short-term investments
Amortized Cost	21,517	37,181
Gross Unrealized Losses	(7)	(77)
Estimated Fair Value	21,510	37,104
Certificates of deposit
Short-term investments
Amortized Cost	43,421	48,551
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 43,421	$ 48,551

Fair Value Measurements (Details) - Fair value measured on recurring basis - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Warrants
Liabilities:
Warrants	$ 581	$ 632
Level 3 \| Warrants
Liabilities:
Warrants	581	632
Municipal bonds and notes
Assets:
Available-for-sale securities	8,787	16,790
Municipal bonds and notes \| Level 2
Assets:
Available-for-sale securities	8,787	16,790
Zero coupon bonds
Assets:
Available-for-sale securities		499
Zero coupon bonds \| Level 2
Assets:
Available-for-sale securities		499
Corporate bonds
Assets:
Available-for-sale securities	21,510	37,104
Corporate bonds \| Level 2
Assets:
Available-for-sale securities	21,510	37,104
Certificates of deposit
Assets:
Available-for-sale securities	43,421	48,551
Certificates of deposit \| Level 2
Assets:
Available-for-sale securities	$ 43,421	$ 48,551

Fair Value Measurements - Fair Value Recognition (Details) - Series I Warrant Liabilities - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the beginning and ending balances of items measured at fair value using significant unobservable inputs (Level 3)
Beginning Balance	$ 632	$ 2,046
Gain included in earnings	(51)	(1,414)
Ending Balance	$ 581	$ 632

Other Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Other Receivables
Other receivables	$ 21,934	$ 60,667
Loans to customers to finance vehicle purchases
Other Receivables
Other receivables	7,416	10,531
Accrued customer billings
Other Receivables
Other receivables	4,308	7,106
Fuel tax and carbon credits
Other Receivables
Other receivables	6,358	40,730
Other
Other Receivables
Other receivables	$ 3,852	$ 2,300

Land, Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Land, Property and Equipment
Land, property and equipment, gross	$ 752,407	$ 733,834
Less: accumulated depreciation	(268,484)	(217,510)
Land, property and equipment, net	483,923	516,324
Capitalized software costs, net	25,728	22,886
Accumulated amortization on the capitalized software costs	17,237	13,793
Amortization expense related to the capitalized software costs	3,444	3,053	$ 2,993
Amount included in accounts payable balances	4,053	5,955
Land
Land, Property and Equipment
Land, property and equipment, gross	2,858	2,858
LNG liquefaction plants
Land, Property and Equipment
Land, property and equipment, gross	94,634	94,634
RNG plants
Land, Property and Equipment
Land, property and equipment, gross	47,545	46,397
Station equipment
Land, Property and Equipment
Land, property and equipment, gross	341,605	316,258
Trailers
Land, Property and Equipment
Land, property and equipment, gross	54,985	50,414
Other equipment
Land, Property and Equipment
Land, property and equipment, gross	93,118	83,687
Construction in progress
Land, Property and Equipment
Land, property and equipment, gross	$ 117,662	$ 139,586

Investment in Other Entities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 16, 2014
Equity method investments
Loss from investment	$ (22)	$ (815)	$ (490)
Capital from equity method investment	$ 3,031	0	0
RNG Ventures
Equity method investments
Ownership interest	50.00%
Responsibility percent of development costs	50.00%
Percentage of right to purchase products for vehicle fuels market	100.00%
Investment balance	$ 833
Mansfield Energy Corp
Equity method investments
Ownership interest				50.00%
Investment balance	1,642	4,695
Loss from investment	22	$ 815	$ 490
Capital from equity method investment	$ 3,031

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Accrued alternative fuels incentives	$ 9,840	$ 15,651
Accrued employee benefits	4,317	3,042
Accrued interest	1,849	3,718
Accrued gas and equipment purchases	11,657	14,133
Accrued property and other taxes	4,572	5,344
Salaries and wages	12,293	9,537
Other	8,073	7,657
Total accrued liabilities	$ 52,601	$ 59,082

Debt - Debt and Capital Lease Obligations (Details) - USD ($)	Dec. 31, 2016	Feb. 29, 2016	Dec. 31, 2015	Sep. 30, 2013	Aug. 24, 2011	Jul. 11, 2011
Long-term debt
Principal Balances	$ 314,348,000		$ 572,414,000
Unamortized Debt Financing Costs	1,972,000		5,264,000
Balance, Net of Financing Costs	312,376,000		567,150,000
Principal Balances, amounts due within one year	(6,126,000)		(150,129,000)
Unamortized Debt Financing Costs, amounts due within one year	(183,000)		(273,000)
Balance, Net of Financing Costs, amounts due within one year	(5,943,000)		(149,856,000)
Principal Balances, long-term	308,222,000		422,285,000
Unamortized Debt Financing Costs, long-term	1,789,000		4,991,000
Balance, Net of Financing Costs, long-term	306,433,000		417,294,000
Summary of aggregate maturities of long-term debt
2017	6,126,000
2018	189,646,000
2019	55,493,000
2020	54,634,000
2021	3,996,000
Thereafter	4,453,000
7.5% Notes
Long-term debt
Principal Balances	150,000,000		150,000,000
Unamortized Debt Financing Costs	274,000		399,000
Balance, Net of Financing Costs	149,726,000		149,601,000
Interest rate						7.50%
Summary of aggregate maturities of long-term debt
2017	0
2018	50,000,000
2019	50,000,000
2020	50,000,000
2021	0
Thereafter	0
7.5% Notes \| Long-Term Debt, Related Party \| Pickens
Long-term debt
Balance, Net of Financing Costs	65,000,000
SLG Notes
Long-term debt
Principal Balances			145,000,000
Unamortized Debt Financing Costs			38,000
Balance, Net of Financing Costs			144,962,000
Interest rate					7.50%
5.25% Notes
Long-term debt
Principal Balances	110,450,000		250,000,000
Unamortized Debt Financing Costs	1,088,000		3,985,000
Balance, Net of Financing Costs	109,362,000		246,015,000
Interest rate				5.25%
Summary of aggregate maturities of long-term debt
2017	0
2018	110,450,000
2019	0
2020	0
2021	0
Thereafter	0
Plains Capital Bank Credit Agreement
Long-term debt
Principal Balances	23,500,000
Unamortized Debt Financing Costs	0
Balance, Net of Financing Costs	23,500,000
Interest rate		2.70%
Summary of aggregate maturities of long-term debt
2017	0
2018	23,500,000
2019	0
2020	0
2021	0
Thereafter	0
Canton Bonds
Long-term debt
Principal Balances	9,520,000		10,910,000
Unamortized Debt Financing Costs	373,000		514,000
Balance, Net of Financing Costs	9,147,000		10,396,000
Summary of aggregate maturities of long-term debt
2017	1,420,000
2018	1,460,000
2019	1,555,000
2020	1,665,000
2021	1,695,000
Thereafter	1,725,000
Capital lease obligations
Long-term debt
Principal Balances	6,028,000		6,448,000
Unamortized Debt Financing Costs	0		0
Balance, Net of Financing Costs	6,028,000		6,448,000
Summary of aggregate maturities of long-term debt
2017	1,890,000
2018	1,569,000
2019	1,568,000
2020	462,000
2021	265,000
Thereafter	274,000
Other debt
Long-term debt
Principal Balances	14,850,000		10,056,000
Unamortized Debt Financing Costs	237,000		328,000
Balance, Net of Financing Costs	14,613,000		$ 9,728,000
Summary of aggregate maturities of long-term debt
2017	2,816,000
2018	2,667,000
2019	2,370,000
2020	2,507,000
2021	2,036,000
Thereafter	$ 2,454,000

Debt - 7.5% Notes (Details) - 7.5% Notes	Jun. 14, 2013 USD ($) day $ / shares	Jul. 11, 2011 USD ($) note	Aug. 27, 2013 USD ($)
Long-term debt
Interest rate		7.50%
Financing commitment received (up to)		$ 150,000,000
Number of convertible promissory notes to be issued \| note		3
Financing commitment issuance period		3 years
Principal amount of each debt instrument to be issued		$ 50,000,000
Pickens
Long-term debt
Aggregate principal amount			$ 65,000,000
Buyers
Long-term debt
Additional amount of advances under the obligation assumed	$ 50,000,000
Amount of advance funded	$ 50,000,000
Conversion price of shares (in dollars per share) \| $ / shares	$ 15.80
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion	40.00%
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion \| day	20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion	30 days
Period during which the debt instrument principal balance is required to be paid following its issuance	7 years
Green Energy Investment Holdings, LLC
Long-term debt
Principal amount transferred			5,000,000
Aggregate principal amount			80,000,000
Other Third Parties
Long-term debt
Aggregate principal amount			$ 5,000,000

Debt - SLG Notes (Details) - SLG Notes - USD ($)	Jul. 14, 2016	Feb. 29, 2016	Mar. 01, 2012	Aug. 24, 2011
Long-term debt
Financing commitment received				$ 150,000,000
Interest rate				7.50%
Principal amount transferred			$ 24,000,000
Repayments of debt		$ 60,000,000
Repayments of debt, accrued interest		$ 1,812,000
Purchaser
Long-term debt
Repayments of debt, accrued interest	$ 248,000
Debt instrument, repurchased face amount	85,000,000
Repayments of convertible debt	38,155,000
Gain (loss) on repurchase of debt instrument	$ (891,000)
Purchaser \| Common stock
Long-term debt
Common stock issued upon conversion of debt (in shares)	14,000,000

Debt - 5.25% (Details)		1 Months Ended	9 Months Ended	12 Months Ended
Debt - 5.25% (Details)	May 04, 2016 USD ($) shares	Sep. 30, 2013 USD ($) day $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Long-term debt
Payment of certain debt issuance costs				$ 0	$ 0	$ 896,000
5.25% Notes
Long-term debt
Interest rate		5.25%
Debt issuance amount		$ 250,000,000
Payment of certain debt issuance costs		7,805,000
Amount of advance funded		242,195,000
Denominator in calculation of shares per principal amount in debt conversion		$ 1,000
Common stock issued upon conversion of debt (in shares) \| shares	6,265,829	64.1026
Conversion price of shares (in dollars per share) \| $ / shares		$ 15.60
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion \| day		20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion		30 days
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion		160.00%
Redemption price as percentage of principal amount of notes to be redeemed		100.00%
Percentage of principal amount at which notes may be required to be repurchased in event of fundamental change by the entity		100.00%
Repayments of convertible debt				84,344,000
Repurchased face amount during period				114,550,000
Repayments of debt, accrued interest			$ 1,546,000
Converted instrument, amount	$ 25,000,000
Gain (loss) on repurchase of debt instrument				$ 35,239,000
5.25% Notes \| Minimum
Long-term debt
Percentage of principal amount of notes outstanding allowing holders to accelerate all amounts due under notes in event of default under the Indenture		25.00%

Debt - PlainsCapital Bank Credit Facility (Details) - Plains Capital Bank Credit Agreement - USD ($)	Feb. 29, 2016	Dec. 31, 2016
Long-term debt
Line of credit limit (up to)	$ 50,000,000
Period during which the debt instrument principal balance is required to be paid following its issuance	1 year
Draw down of line of credit	$ 50,000,000	$ 23,500,000
Interest rate	2.70%
London Interbank Offered Rate (LIBOR
Long-term debt
Basis spread on variable rate	2.30%

- Definition

The carrying value as of the balance sheet date of the noncurrent portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References + Details

Debt - Canton Bonds and Other Debt (Details) - USD ($)	12 Months Ended
Debt - Canton Bonds and Other Debt (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Mar. 19, 2014
Long-term debt
Maximum effective interest rate (up to)	21.45%
Weighted average interest rate	5.00%	6.35%
Canton Bonds
Long-term debt
Principal amount with recourse to the Company	$ 1,000,000
Canton Bonds \| Canton Renewables
Long-term debt
Debt issuance amount			$ 12,400,000
Coupon interest rate			6.60%

Debt - Termination of GE Loans and Marvix Note (Details)				3 Months Ended	12 Months Ended
	Dec. 29, 2014	Apr. 25, 2013 USD ($)	Nov. 07, 2012 USD ($) subsidiary plant $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)
Long-term debt
Number of shares that will not vest and will not become exercisable (in shares) \| shares				4,000,000	4,000,000
GE Credit Agreement
Long-term debt
Number of wholly owned subsidiaries through which the entity entered into a financing arrangement \| subsidiary			2
Line of credit limit (up to)			$ 200,000,000
Number of LNG production facilities being financed \| plant			2
Commitment fee on the unutilized loan amounts			0.50%
Commitment fee					$ 1,014,000	$ 1,014,000
Project completion period, months after funding	30 months
Write off of deferred financing costs				$ 54,925,000
GE Credit Agreement \| GE Warrant \| Common stock
Long-term debt
Number of shares that can be purchased upon exercise of warrants (in shares) \| shares			5,000,000
Exercise price of the warrant (in dollars per share) \| $ / shares			$ 0.01
Number of shares that will not vest and will not become exercisable (in shares) \| shares				4,000,000	4,000,000
Mavrix Note \| Note Purchaser
Long-term debt
Aggregate principal amount		$ 30,000,000
Amount funded		15,000,000
Repayments of debt		13,594,000
Early termination fee		$ 750,000

Derivative Transactions (Details) - USD ($)	12 Months Ended
Derivative Transactions (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Transactions
Derivative gain (loss)	$ 22,000	$ 1,414,000	$ 5,748,000
Forward exchange contracts
Derivative Transactions
Number of commodity futures or forward exchange contracts outstanding	$ 0	$ 0
Natural gas futures contracts
Derivative Transactions
Unrealized gains on derivative instruments recognized in other comprehensive income loss			108,000
Derivative gain (loss)			$ (65,000)

Stockholders' Equity (Details)	Dec. 31, 2016 class vote $ / shares shares	Dec. 31, 2015 $ / shares shares
Stockholders' Equity Note [Abstract]
Authorized number of classes of capital stock \| class	2
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001	$ 0.0001
Preferred stock, par value (in dollars per share) \| $ / shares	$ 0.0001	$ 0.0001
Authorized shares (in shares)	225,000,000
Common stock, authorized (in shares)	224,000,000	224,000,000
Preferred stock, authorized (in shares)	1,000,000	1,000,000
Number of votes per share held by holders of common stock \| vote	1

Stockholders' Equity - Issuance of Common Stock and Warrants (Details) - USD ($)				2 Months Ended	3 Months Ended	12 Months Ended
	Oct. 04, 2016	Nov. 07, 2012	Nov. 03, 2008	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 21, 2016	Sep. 09, 2016	Nov. 11, 2015
Stockholders' equity
Common stock, issued (in shares)				92,382,717	92,382,717	145,538,063	92,382,717
ATM Program Activity
Fees and issuance costs						$ 2,283,000	$ 883,000	$ 0
Equity distribution agreement \| Citigroup
Stockholders' equity
Common stock, amount authorized (up to)									$ 200,000,000	$ 110,000,000	$ 75,000,000
ATM Program Activity
Gross proceeds				$ 6,943,000		103,591,000
Fees and issuance costs				493,000		2,612,000
Net proceeds				$ 6,450,000		$ 100,979,000
Shares issued (in shares)				1,561,902		31,064,434
GE Credit Agreement
Stockholders' equity
Write off of deferred financing costs					$ 54,925,000
Series I Warrants
Stockholders' equity
Period from which warrants are exercisable			6 months
Warrant expiration term			7 years
Exercise price of the warrant (in dollars per share)			$ 12.54
GE Warrant \| GE Credit Agreement
Stockholders' equity
Warrants outstanding, not exercisable (in shares)				4,000,000	4,000,000		4,000,000
Common stock, issued (in shares)	997,740
Fair value of warrants issued, additional paid-in capital		$ 56,158,000
GE Warrant \| Common stock \| GE Credit Agreement
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants (in shares)		5,000,000
Exercise price of the warrant (in dollars per share)		$ 0.01
Number of securities called by warrants (in shares)	1,000,000
NG Advantage Warrants \| NG Advantage
ATM Program Activity
Fair value of warrants outstanding				$ 561,000	$ 561,000	$ 581,000	$ 561,000
Gain from the change in fair value						$ (21,000)	$ 69,000	$ 0
NG Advantage Warrants \| Common stock \| NG Advantage
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants (in shares)						127,200
Maximum \| Series I Warrants
Stockholders' equity
Number of shares issued for units under Placement Agent Agreement (in shares)			4,419,192
Number of shares that can be purchased upon exercise of warrants (in shares)			3,314,394

Stockholders' Equity - Stock-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-based compensation
Stock-based compensation expense, net of $0 tax in 2014, 2015 and 2016	$ 8,092,000	$ 10,779,000	$ 11,514,000
Stock-based compensation expense, tax	$ 0	$ 0	$ 0
2006 Plan \| Stock Options
Stock-based compensation
Number of shares available for future grant	6,112,582
Number of Shares
Outstanding at the beginning of the period (in shares)	11,487,938	11,486,301	11,526,998
Granted (in shares)	284,750	1,415,200	957,000
Exercised (in shares)	0	(608,279)	(468,279)
Forfeited or expired (in shares)	(304,892)	(805,284)	(529,418)
Outstanding at the end of the period (in shares)	11,467,796	11,487,938	11,486,301
Exercisable at the end of the period (in shares)	10,176,324
Vested and expected to vest at the end of the period (in shares)	11,351,637
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 11.44	$ 11.91	$ 11.79
Granted (in dollars per share)	3.63	5.39	10.23
Exercised (in dollars per share)	0.00	2.96	3.78
Forfeited or expired (in dollars per share)	11.30	14.04	13.40
Outstanding at the end of the period (in dollars per share)	11.25	$ 11.44	$ 11.91
Exercisable at the end of the period (in dollars per share)	11.96
Vested and expected to vest at the end of the period (in dollars per share)	$ 11.30
Additional option disclosures
Outstanding at the end of the period, weighted average remaining contractual term	3 years 11 months 27 days
Exercisable at the end of the period, weighted average remaining contractual term	3 years 4 months 28 days
Vested and expected to vest at the end of the period, weighted average remaining contractual term	3 years 10 months 17 days
Outstanding at the end of the period, aggregate intrinsic value	$ 0
Exercisable at the end of the period, aggregate intrinsic value	0
Vested and expected to vest at the end of the period, aggregate intrinsic value	0
Other disclosures
Total unrecognized compensation cost related to non-vested shares	$ 2,794,000
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized	1 year 8 months 5 days
Total fair value of shares vested	$ 2,544,000
Intrinsic value of all options exercised	$ 0	$ 2,197,000	$ 2,568,000
Weighted-average assumption used for grants
Dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	61.10%	59.20%	52.30%
Expected volatility, maximum	70.80%	72.00%	67.00%
Risk-free interest rate, minimum	1.20%	1.70%	1.10%
Risk-free interest rate, maximum	2.00%	1.80%	1.80%
Expected life	6 years	6 years	6 years
Stock-based compensation
Weighted-average grant date fair values of options granted (in dollars per share)	$ 2.30	$ 3.29	$ 5.32
Stock-based compensation expense	$ 2,561,000	$ 5,195,000	$ 7,286,000

Stockholders' Equity - Restricted Stock Units (Details) - USD ($)		12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Market-Based RSUs
Stock-based compensation
Number of shares of common stock to be received for each performance unit (in shares)		1
Period from the date of grant after which awards will be automatically forfeited if the Stock Price Condition is not satisfied		4 years
Shares forfeited (in shares)	1,605,500
Number of Shares
Outstanding at the beginning of the period (in shares)		1,769,000	1,769,000	1,545,000
Granted (in shares)		0	0	489,500
Vested (in shares)		0	0	0
Forfeited or expired (in shares)		(1,340,000)	0	(265,500)
Outstanding at the end of the period (in shares)	429,000	429,000	1,769,000	1,769,000
Expected to vest at the end of the period (in shares)	0	0
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)		$ 10.67	$ 10.67	$ 11.42
Granted (in dollars per share)		0.00	0.00	8.26
Vested (in dollars per share)		0.00	0.00	0.00
Forfeited or expired (in dollars per share)		11.44	0.00	10.62
Outstanding at the end of the period (in dollars per share)	$ 8.26	8.26	$ 10.67	$ 10.67
Expected to vest at the end of the period (in dollars per share)	$ 0.00	$ 0.00
Outstanding at the end of the period, weighted average remaining contractual term		1 year 1 month 2 days
Expected to vest at the end of the period, weighted average remaining contractual term		0 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 0	$ 0
Stock-based compensation expense		$ 169,000	$ 1,770,000	$ 2,556,000
Market-Based RSUs \| Minimum
Stock-based compensation
Period from the date of grant in which the closing price of the entity's common stock must exceed the closing price in order for a holder to receive one share of common stock for each award		2 years
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the closing price on the grant date in order for a holder to receive one share of common stock for each award		20 days
Percentage of the closing price of the entity's common stock that the closing price must equal or exceed in order for an award holder to receive one share of common stock for each award		135.00%
Market-Based RSUs \| Maximum
Stock-based compensation
Period from the date of grant in which the closing price of the entity's common stock must exceed the closing price in order for a holder to receive one share of common stock for each award		4 years
Service-Based RSUs
Number of Shares
Outstanding at the beginning of the period (in shares)		1,650,776	822,752	45,836
Granted (in shares)		850,125	1,167,750	792,500
Vested (in shares)		(726,687)	(283,726)	(15,584)
Forfeited or expired (in shares)		(130,910)	(56,000)	0
Outstanding at the end of the period (in shares)	1,643,304	1,643,304	1,650,776	822,752
Expected to vest at the end of the period (in shares)	1,545,222	1,545,222
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)		$ 5.50	$ 5.82	$ 13.09
Granted (in dollars per share)		3.63	5.38	5.54
Vested (in dollars per share)		5.53	5.94	13.09
Forfeited or expired (in dollars per share)		4.91	5.57	0.00
Outstanding at the end of the period (in dollars per share)	$ 4.56	4.56	$ 5.50	$ 5.82
Expected to vest at the end of the period (in dollars per share)	$ 4.56	$ 4.56
Outstanding at the end of the period, weighted average remaining contractual term		1 year 2 months 9 days
Expected to vest at the end of the period, weighted average remaining contractual term		11 months 27 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 5,799,000	$ 5,799,000
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized		1 year 2 months 16 days
Stock-based compensation expense		$ 4,395,000	$ 2,622,000	$ 365,000
Vesting period		3 years
Service-Based RSUs \| Vesting over the first year
Stock-based compensation
Vesting percentage		34.00%
Service-Based RSUs \| Vesting over the second year
Stock-based compensation
Vesting percentage		33.00%
Service-Based RSUs \| Vesting over the third year
Stock-based compensation
Vesting percentage		33.00%
2012 and 2014 \| Market-Based RSUs
Number of Shares
Granted (in shares)				2,034,500	2,034,500

Stockholders' Equity - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan $ in Thousands		12 Months Ended			44 Months Ended
	May 07, 2013 period	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 shares
Stock-based compensation
Purchase price of shares expressed as percentage of fair market value of common stock	85.00%
Number of non-concurrent offering periods \| period	2
Number of shares reserved (in shares)		2,500,000			2,500,000
Stock-based compensation expense \| $		$ 51	$ 50	$ 67
Shares sold pursuant to the ESPP (in shares)					199,884

Stockholders' Equity - Non-Qualified Non-Public Subsidiary Unit Options (Details) - CERF Plan - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Shares
Outstanding at the beginning of the period (in shares)		108,000	115,000	115,000
Granted (in shares)		0	0	0
Exercised (in shares)		0	0	0
Forfeited or expired (in shares)		0	(7,000)	0
Outstanding at the end of the period (in shares)		108,000	108,000	115,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 40.80	$ 40.80	$ 40.80
Granted (in dollars per share)		0.00	0.00	0.00
Exercised (in dollars per share)		0.00	0.00	0.00
Forfeited or expired (in dollars per share)		0.00	40.80	0.00
Outstanding at the end of the period (in dollars per share)		$ 40.80	$ 40.80	$ 40.80
Additional option disclosures
Outstanding at the end of the period, weighted average remaining contractual term		6 years 8 months 19 days
Outstanding at the end of the period, aggregate intrinsic value		$ 94,620.00
Stock-based compensation
Weighted-average grant date fair values of options granted (in dollars per share)	$ 31.65
Stock Options
Stock-based compensation
Number of shares reserved for issuance	150,000
Other disclosures
Total unrecognized compensation cost related to non-vested units		0
Stock-based compensation
Stock-based compensation expense		$ 803,000	$ 1,115,000	$ 1,240,000

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of loss before income taxes
U.S.	$ 7,150	$ (111,437)	$ (64,913)
Foreign	(19,535)	(22,407)	(24,871)
Total loss before income taxes	(12,385)	(133,844)	(89,784)
Current:
Federal	(226)	9	190
State	93	248	238
Foreign	567	912	1,017
Total current	434	1,169	1,445
Deferred:
Federal	478	337	29
State	75	71	(10)
Foreign	352	37	(389)
Total deferred	905	445	(370)
Income tax expense (benefit)	$ 1,339	$ 1,614	$ 1,075
Reconciliation of the income tax provision
Federal income tax rate	35.00%	35.00%	35.00%
Computed expected tax (benefit)	$ (4,335)	$ (46,846)	$ (30,415)
Nondeductible expenses	5,971	24,998	10,690
Tax rate differential on foreign earnings	720	3,701	5,733
Tax credits	(9,331)	(9,988)	(8,286)
Other	833	(372)	(1,121)
Change in valuation allowance	7,481	30,121	24,474
Income tax expense (benefit)	1,339	1,614	1,075
Tax benefit related to exclusion of VETC associated with 2013 fuel sales	9,112	9,298	$ 8,221
Deferred tax assets:
Accrued expenses	4,566	4,512
Sales-type leases	55	154
Alternative minimum tax and general business credits	6,137	5,780
Stock option expense	26,154	23,113
Other	1,168	2,283
Loss carryforwards	181,884	174,157
Total deferred tax assets	219,964	209,999
Less valuation allowance	(195,968)	(189,203)
Net deferred tax assets	23,996	20,796
Deferred tax liabilities:
Depreciation and amortization	(19,364)	(17,398)
Goodwill	(5,599)	(4,600)
Partnership income	(1,432)	(293)
Total deferred tax liabilities	(26,395)	(22,291)
Net deferred tax liabilities	$ (2,399)	$ (1,495)

Income Taxes - Tax Credit Carryforwards (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Federal
Income Taxes
Operating loss carryforwards	$ 447,788
Federal \| General
Income Taxes
Tax credit carryforwards	5,907
State
Income Taxes
Operating loss carryforwards	329,058
Foreign
Income Taxes
Operating loss carryforwards	$ 79,148

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation allowance
Valuation allowance	$ 195,968	$ 189,203
Net increase (decrease) in valuation allowance	6,765	28,767
Undistributed earnings of non-U.S. subsidiaries	4,568
Unrecognized tax benefits, which if recognized, would primarily affect the effective tax rate	692
Unrecognized tax benefits, which if recognized, would increase the net operating loss carryforwards	48,910
Reconciliation of the total amounts of unrecognized tax benefits
Unrecognized tax benefit at the beginning of the period	27,497	21,974
Gross increases—tax positions in current year	4,556	5,523
Gross increases—tax positions in prior years	17,549	0
Unrecognized tax benefit at the end of the period	49,602	27,497	$ 21,974
Additional information on income tax
Accrued interest expense and penalties	241	178
Interest expense related to uncertain tax positions	$ 62	$ 58	$ 54

Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fiscal year:
2017	$ 8,055
2018	6,685
2019	6,287
2020	5,575
2021	4,195
Thereafter	17,801
Total future minimum lease payments	48,598
Rent expense	$ 11,058	$ 8,629	$ 10,140

Commitments and Contingencies - Long-Term Take-or-Pay Natural Gas Supply Contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
LNG supply agreement \| DGS
Long-Term Take-or-pay Contracts
Amount paid under the contract	$ 9,692	$ 11,852	$ 14,267
Fixed commitments under the contract payable in future
2017	3,614
2018	3,614
2019	3,614
2020	901
CNG supply agreement \| JTA
Fixed commitments under the contract payable in future
2017	203
2018	313
2019	429
2020	$ 548

Capitalized Lease Obligation and Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Weighted average interest rate	8.70%
Future payments under these capital leases
2017	$ 2,319
2018	1,887
2019	1,777
2020	575
2021	338
Thereafter	295
Total minimum lease payments	7,191
Less amount representing interest	(1,163)
Present value of future minimum lease payments	6,028
Less current portion	(1,890)
Capital lease obligations, less current portion	4,138
Value of the equipment under capital lease	10,168	$ 8,970
Accumulated amortization	$ 4,073	$ 2,766

Capitalized Lease Obligation and Receivables - Future Receipts (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Leases [Abstract]
Sales-type leases interest rate	12.10%
Future receipts under the leases
2017	$ 380
2018	251
2019	186
2020	186
2021	186
Thereafter	1,612
Total	2,801
Less amount representing interest	(1,394)
Present value of future minimum lease receipts	$ 1,407

401(k) Plan (Details) - USD ($) $ in Thousands	12 Months Ended
401(k) Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Maximum percentage of base pay that can be contributed by employees through salary deferrals	90.00%
Contribution by the company	$ 1,527	$ 304	$ 1,040

Geographic Information (Details) - USD ($) $ in Thousands	12 Months Ended
Geographic Information (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Geographic Information
Total revenue	$ 402,656	$ 384,320	$ 428,940
Total operating income (loss)	(17,637)	(41,623)	(54,364)
Total long-lived assets	619,116	656,629	674,866
United States
Geographic Information
Total revenue	378,497	330,003	360,881
Total operating income (loss)	(8,693)	(33,067)	(41,543)
Total long-lived assets	547,279	582,644	582,028
Canada
Geographic Information
Total revenue	11,502	21,818	16,241
Total operating income (loss)	(4,212)	(4,980)	(3,087)
Total long-lived assets	66,191	68,292	85,984
Other
Geographic Information
Total revenue	12,657	32,499	51,818
Total operating income (loss)	(4,732)	(3,576)	(9,734)
Total long-lived assets	$ 5,646	$ 5,693	$ 6,854

Concentrations (Details) - supplier	12 Months Ended
Concentrations (Details) - supplier	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Natural gas expense \| Supplier concentration
Supplier Concentrations
Concentration risk, number of suppliers	4	3	2

Subsequent Events (Details) - USD ($)	Feb. 27, 2017	Feb. 06, 2017	Dec. 31, 2016	Aug. 27, 2013	Jul. 11, 2011
7.5% Notes
Subsequent Event [Line Items]
Interest rate					7.50%
Pickens \| 7.5% Notes
Subsequent Event [Line Items]
Aggregate principal amount				$ 65,000,000
RNG Ventures
Subsequent Event [Line Items]
Ownership interest			50.00%
Subsequent Event \| Pickens \| 7.5% Notes
Subsequent Event [Line Items]
Interest rate		7.50%
Aggregate principal amount		$ 25,000,000
Repayments of convertible debt		$ 21,750,000
Subsequent Event \| BP Products North America, Inc.
Subsequent Event [Line Items]
Asset purchase agreement, amount	$ 155,000,000
Asset purchase agreement, contingent consideration (up to)	25,000,000
Asset purchase agreement, payment of transaction costs	30,000,000
Asset purchase agreement, debt instrument, face amount	$ 125,000,000

Schedule II: Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowances for Doubtful Trade Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	$ 1,895	$ 752	$ 832
Charges (benefit) to operations	1,107	1,514	387
Deductions	(1,939)	(371)	(467)
Balance at the end of the period	1,063	1,895	752
Allowance for Doubtful Notes Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	3,990	2,850	2,416
Charges (benefit) to operations	1,617	1,142	890
Deductions	(4,377)	(2)	(456)
Balance at the end of the period	$ 1,230	$ 3,990	$ 2,850