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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2015	Mar. 01, 2016	Jun. 30, 2015
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, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		97,360,841
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 399,840,166

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 43,724	$ 92,381
Restricted cash	4,240	6,012
Short-term investments	102,944	122,546
Accounts receivable, net of allowance for doubtful accounts of $752 and $1,895 as of December 31, 2014 and December 31, 2015, respectively	73,645	81,970
Other receivables	60,667	56,223
Inventory	29,289	34,696
Prepaid expenses and other current assets	14,930	19,811
Total current assets	329,439	413,639
Land, property and equipment, net	516,324	514,269
Notes receivable and other long-term assets, net	19,723	71,904
Investments in other entities	5,695	6,510
Goodwill	91,967	98,726
Intangible assets, net	42,644	55,361
Total assets	1,005,792	1,160,409
Current liabilities:
Current portion of debt and capital lease obligations	150,129	4,846
Accounts payable	26,906	43,922
Accrued liabilities	59,082	56,760
Deferred revenue	10,549	14,683
Total current liabilities	246,666	120,211
Long-term portion of debt and capital lease obligations	357,285	500,824
Long-term debt, related party	65,000	65,000
Other long-term liabilities	7,896	9,339
Total liabilities	676,847	695,374
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 90,203,344 shares and outstanding 92,382,717 shares at December 31, 2014 and December 31, 2015, respectively	9	9
Additional paid-in capital	915,199	898,106
Accumulated deficit	(591,683)	(457,441)
Accumulated other comprehensive income (loss)	(20,973)	(3,248)
Total Clean Energy Fuels Corp. stockholders' equity	302,552	437,426
Noncontrolling interest in subsidiary	26,393	27,609
Total stockholders' equity	328,945	465,035
Total liabilities and stockholders' equity	$ 1,005,792	$ 1,160,409

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,895	$ 752
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, Authorized shares	1,000,000	1,000,000
Preferred stock, issued shares	0	0
Preferred stock, outstanding shares	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, Authorized shares	224,000,000	224,000,000
Common stock, issued shares	92,382,717	90,203,344
Common stock, outstanding shares	92,382,717	90,203,344

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue:
Product revenues	$ 329,168	$ 380,199	$ 310,813
Service revenues	55,152	48,741	41,662
Total revenue	384,320	428,940	352,475
Cost of sales (exclusive of depreciation and amortization shown separately below):
Product cost of sales	230,621	291,462	213,593
Service cost of sales	27,864	17,325	11,169
Gain from change in fair value of derivative warrants	(1,414)	(5,748)	(938)
Selling, general and administrative	113,653	126,435	138,024
Depreciation and amortization	55,219	49,058	42,318
Impairment of long-lived asset	0	4,772	0
Total operating expenses	425,943	483,304	404,166
Operating loss	(41,623)	(54,364)	(51,691)
Interest expense, net	(94,970)	(44,357)	(29,287)
Other income (expense), net	2,627	(2,571)	(970)
Loss from equity method investments	(815)	(490)	(76)
Gain from sale of equity method investment	0	0	4,705
Gain from sale of subsidiary	937	11,998	14,115
Loss before income taxes	(133,844)	(89,784)	(63,204)
Income tax expense	(1,614)	(1,075)	(3,715)
Net loss	(135,458)	(90,859)	(66,919)
Loss (income) of noncontrolling interest	1,216	1,200	(49)
Net loss attributable to Clean Energy Fuels Corp.	$ (134,242)	$ (89,659)	$ (66,968)
Loss per share:
Basic and diluted (in dollars per share)	$ (1.47)	$ (0.96)	$ (0.71)
Weighted average common shares outstanding:
Basic and diluted (in shares)	91,607,578	93,678,432	93,958,758

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income (loss)	$ (135,458)	$ (90,859)	$ (66,919)
Net Income (Loss) Available to Common Stockholders, Basic	(134,242)	(89,659)	(66,968)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2013, 2014 and 2015	(9,653)	(7,958)	(1,680)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2013, 2014 and 2015	(8,078)	4,866	(4,834)
Unrealized gains (losses) on available-for sale securities, net of $0 tax in 2013, 2014, and 2015	6	544	(445)
Unrecognized gains on derivatives, net of $0 tax in 2013, 2014 and 2015	0	0	108
Total other comprehensive income (loss)	(17,725)	(2,548)	(6,851)
Comprehensive income (loss)	(153,183)	(93,407)	(73,770)
Clean Energy Fuels Corp
Net income (loss)	(134,242)	(89,659)	(66,968)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2013, 2014 and 2015	(9,653)	(7,958)	(1,680)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2013, 2014 and 2015	(8,078)	4,866	(4,834)
Unrealized gains (losses) on available-for sale securities, net of $0 tax in 2013, 2014, and 2015	6	544	(445)
Unrecognized gains on derivatives, net of $0 tax in 2013, 2014 and 2015	0	0	108
Total other comprehensive income (loss)	(17,725)	(2,548)	(6,851)
Comprehensive income (loss)	(151,967)	(92,207)	(73,819)
Noncontrolling Interest in Subsidiary
Net income (loss)	(1,216)	(1,200)	49
Other comprehensive income (loss), net of tax:
Comprehensive income (loss)	$ (1,216)	$ (1,200)	$ 49

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (PARENTHETICAL) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Subsidiary
Beginning balance (in shares) at Dec. 31, 2012		87,634,478
Beginning balance at Dec. 31, 2012	$ 546,630	$ 9	$ 837,367	$ (300,814)	$ 6,151	$ 3,917
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of applicable taxes (in shares)		119,349
Issuance of common stock, net of applicable taxes	677		677
Issuance of common stock, net of offering costs (in shares)		1,610,570
Issuance of common stock, net of offering costs	21,993		21,993
Stock-based compensation	23,008		23,008
Net Income (Loss) Available to Common Stockholders, Basic	(66,968)
Net income (loss)	(66,919)			(66,968)		49
Accumulated other comprehensive loss	(6,851)				(6,851)
Ending balance (in shares) at Dec. 31, 2013		89,364,397
Ending 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 applicable taxes (in shares)		519,608
Issuance of common stock, net of applicable taxes	2,300		2,300
Issuance of common stock, net of offering costs (in shares)		319,339
Issuance of common stock, net of offering costs	3,750		3,750
Exercise of additional membership interest in subsidiary	2,363		2,363
Stock-based compensation	11,514		11,514
Net Income (Loss) Available to Common Stockholders, Basic	(89,659)
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 income (loss)	(90,859)			(89,659)		(1,200)
Accumulated other comprehensive loss	$ (2,548)				(2,548)
Ending balance (in shares) at Dec. 31, 2014	90,203,344	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
Issuance of common stock, net of applicable taxes (in shares)		2,179,373
Issuance of common stock, net of applicable taxes	6,314		6,314
Stock-based compensation	10,779		10,779
Net Income (Loss) Available to Common Stockholders, Basic	(134,242)
Net income (loss)	(135,458)			(134,242)		(1,216)
Accumulated other comprehensive 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

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (135,458)	$ (90,859)	$ (66,919)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	55,219	49,058	42,318
Provision for doubtful accounts, notes and inventory	2,656	1,277	1,962
Derivative gain	(1,414)	(5,748)	(938)
Stock-based compensation expense	10,779	11,514	23,008
Amortization of debt issuance cost	2,969	4,194	1,662
Non-cash interest charge related to a terminated credit agreement	54,925	0	0
Accretion of notes payable	57	412	1,140
Gain on sale of equity method investment	0	0	(4,705)
Dividend received on equity method investment	0	0	1,091
Long-lived intangible impairment	0	4,772	0
Gain on sale of subsidiary	(937)	(11,998)	(14,115)
Gain on contingent consideration for acquisitions	0	(208)	(1,132)
Changes in operating assets and liabilities, net of assets and liabilities acquired and disposed:
Accounts and other receivables	3,426	(55,573)	(441)
Inventory	5,407	(979)	(637)
Prepaid expenses and other assets	2,876	1,361	579
Accounts payable	(12,005)	9,126	(6,962)
Accrued expenses and other	(596)	7,646	19,395
Net cash used in operating activities	(12,096)	(76,005)	(4,694)
Cash flows from investing activities:
Purchases of short-term investments	(158,840)	(157,629)	(170,935)
Maturities and sales of short-term investments	176,969	171,902	93,289
Purchases and deposits on property and equipment	(51,415)	(88,628)	(86,730)
Loans made to customers	(4,279)	(9,140)	(3,950)
Payments on and proceeds from sales of loans receivable	928	6,580	4,153
Restricted cash	1,650	(3,567)	13,250
Cash received with sale of subsidiary, net of cash transferred	1,118	39,760	(1,178)
Investments in other entities	0	(6,634)	0
Proceeds from sale of equity method investment	0	0	6,119
Acquisitions, net of cash acquired	0	467	(9,000)
Net cash used in investing activities	(33,869)	(46,889)	(154,982)
Cash flows from financing activities:
Issuances of common stock, net of taxes paid	6,314	2,300	677
Proceeds from debt instruments	384	12,778	300,559
Proceeds from debt, related party	0	0	15,000
Proceeds from revolving line of credit	31	34,607	31,527
Proceeds from exercise of additional membership interest in subsidiary	0	6,992	0
Repayment of borrowings under revolving line of credit	(64)	(40,354)	(37,767)
Repayment of capital lease obligations and debt instruments	(6,258)	(41,036)	(10,147)
Contingent consideration paid relating to business acquisitions	0	(176)	0
Payment for debt issuance costs	0	(896)	(9,130)
Net cash provided (used) by financing activities	407	(25,785)	290,719
Effect of exchange rates on cash and cash equivalents	(3,099)	1,027	468
Net increase (decrease) in cash and cash equivalents	(48,657)	(147,652)	131,511
Cash and cash equivalents, beginning of year	92,381	240,033	108,522
Cash and cash equivalents, end of year	43,724	92,381	240,033
Supplemental disclosure of cash flow information:
Income taxes paid	890	943	2,228
Interest paid, net of $2,517, $3,160, and $835 capitalized, respectively	$ 37,662	$ 39,224	$ 22,110

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Cash Flows [Abstract]
Interest paid, capitalized	$ 835	$ 3,160	$ 2,517

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2015

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 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”) for light, medium and heavy-duty vehicles and providing operation, repair and maintenance ("O&M") services for vehicle fleet customer stations. As a comprehensive solution provider, the Company also designs, builds, operates, services, repairs 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 to large industrial and institutional energy users who do not have direct access to natural gas pipelines, processes and sells RNG, sells tradable credits generated by selling natural gas and RNG as a vehicle fuel, including credits generated under the California and the Oregon Low Carbon Fuel Standards (collectively, "LCFS Credits") and Renewable Identification Numbers ("RIN Credits" or "RINs") generated under the federal Renewable Fuel Standard Phase 2, helps its customers acquire and finance natural gas vehicles and obtains federal, state and local tax credits, grants and incentives.

In addition, through June 28, 2013, the Company provided natural gas vehicle conversions and design and engineering services for natural gas engine systems. Further, 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 consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to state fairly the Company's financial position, results of operations, comprehensive income (loss) and cash flows in accordance with U.S. generally accepted accounting principles ("US GAAP"). All intercompany accounts and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and revenues and expenses recorded during the reporting periods. Actual results could differ from those estimates. 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, short-term investments, accounts and other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable approximate fair value.

Inventories

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, 2014 and 2015:



	2014		2015
Raw materials and spare parts	$	31,389	$	25,113
Work in process	3,292		973
Finished goods	15		3,203
Total	$	34,696	$	29,289

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 $3,114, $959, and $4,292 for the years ended December 31, 2013, 2014, and 2015 respectively.

Long-Lived Assets

The Company reviews long-lived assets, which includes 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.

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 20 years for trademarks and trade names, and three years for non-compete agreements. In 2014, the Company acquired the 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. Also, in the fourth quarter of 2014, the Company determined that a long-lived asset related to a contract acquired in its acquisition of Clean Energy Compression was impaired and recorded an impairment charge of $4,772. The Company had no impairments of any long-lived assets in 2013 or 2015.

The Company's intangible assets as of December 31, 2014 and 2015 were as follows:



	2014			2015
Technology	$	54,400		$	54,400
Customer relationships	16,430			16,576
Acquired contracts	3,694			3,694
Trademark and trade names	8,200			8,200
Non-compete agreements	2,060			2,060
Total intangible assets	84,784			84,930
Less accumulated amortization	(24,384		)	(30,442		)
Foreign currency rate change	(5,039		)	(11,844		)
Net intangible assets	$	55,361		$	42,644

Amortization expense for intangible assets was $9,016, $7,024, and $5,539 for the years ended December 31, 2013, 2014 and 2015, respectively. Estimated amortization expense for the five years and thereafter succeeding the year ended December 31, 2015 is approximately $4,788, $4,574, $4,011, $2,953, $2,933 and $23,385 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 has determined that it is a single reporting unit for the purpose of goodwill impairment tests. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review. During 2013, 2014 and 2015, there were no indicators of impairment to goodwill.

The Company reduced its goodwill balance by $774 and $7,205 when it sold BAF and its interest in DCE and its subsidiary DCEMB on June 28, 2013 and December 29, 2014, respectively, and added $16,555 and $21,070 to its goodwill balance when it acquired MGES and NG Advantage on May 6, 2013 and October 14, 2014, respectively (all as defined and described in note 2). The goodwill balances on the consolidated balance sheets include foreign currency translation gains (losses) of $(3,687) and $(6,578) as of December 31, 2014 and 2015, 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)	2013		2014		2015
Volume related revenue	$	195,328	$	247,899	$	260,629
Clean Energy Compression	77,485		84,775		54,497
Station Construction Project Sales	27,098		67,392		37,830
VETC	45,439		28,359		30,986
Other	7,125		515		378
	$	352,475	$	428,940	$	384,320

The Company’s volume related revenues primarily consist of CNG and LNG fuel sales, RIN and LCFS Credits sales and O&M services. These revenues are recognized in accordance with US GAAP, which requires that the following four criteria must be met before revenue can be recognized: (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 it generates RIN Credits when it sells RNG as a vehicle fuel in the U.S. 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 at the time it has an agreement in place to sell the credits at a fixed or determinable price.

The Company recognizes compression revenues through its subsidiary Clean Energy Compression when it sells non-lubricated natural gas fueling compressors and related 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.

The Company typically recognizes revenue on fueling station construction projects where it sells the station or station upgrade to the customer using the completed-contract method because the projects are short-term and the results of operations reported on the completed-contract basis would not vary materially from those resulting from the use of the percentage-of-completion method. The construction contract is considered to be substantially completed at the earlier of customer acceptance of the fueling station or the time when the fuel dispensing activities begin. When applicable, multi-station construction contracts are segmented into phases as negotiated with customers. Gross margin related to each phase is recognized when it is substantially complete. The Company’s subsidiary Clean Energy Cryogenics, which manufactures components used in the Company’s station sales, 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.

In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction 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 completed-contract 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.

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.

The Company is eligible to receive, at times, a federal alternative fuels tax credit ("VETC") when a gasoline gallon equivalent of CNG and a liquid gallon of LNG is sold as vehicle fuel. 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 refundable and do not need to offset income tax liabilities to be received. See the discussion under “Alternative Fuels Excise Tax Credit” below for further information.

The Company previously owned BAF Technologies, Inc. and its wholly owned subsidiary, ServoTech Engineering, Inc. (BAF Technologies, Inc. and ServoTech Engineering Inc. are collectively referred to as "BAF"), which converted light and medium duty vehicles to run on natural gas and provided design and engineering services for natural gas engine systems. On June 28, 2013, the Company sold BAF to Westport Innovations (U.S.) Holdings Inc., a wholly owned subsidiary of Westport Innovations Inc. As of the sale of BAF, the Company no longer generates revenues from vehicle conversions.

Alternative Fuels Excise Tax Credit

From October 1, 2006 through December 31, 2015, the Company was eligible to receive the VETC alternative fuels tax credit of $0.50 per gasoline gallon equivalent of CNG and $0.50 per liquid gallon of LNG that it sold as vehicle fuel. For 2016, the VETC credit is $0.50 per gasoline gallon equivalent of CNG and $0.50 per diesel gallon equivalent of LNG that is sold as a vehicle fuel. The American Taxpayer Relief Act, signed into law on January 2, 2013, reinstated VETC for 2013 and made it retroactive to January 1, 2012. The Company did not record any VETC revenues in 2012, but recognized VETC revenues in 2013 of $45,439, which included $20,800 for CNG and LNG the Company sold in 2012. The Tax Increase Prevention Act, signed into law on December 19, 2014, reinstated VETC for the 2014 calendar year and made it retroactive to January 1, 2014. As a result, VETC revenues for the 2014 calendar year, totaling $28,359, were all recognized in December 2014. In December 2015, the VETC was extended through December 31, 2016 and made retroactive to January 1, 2015. As a result, VETC revenues for the 2015 calendar year, totaling $30,986, were recognized in December 2015.

LNG Transportation Costs

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

Advertising Costs

Advertising costs are expensed as incurred. Advertising costs amounted to $1,391, $439, and $44 for the years ended December 31, 2013, 2014, and 2015 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.

Foreign Currency Translation

In accordance with FASB authoritative guidance, 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. Net 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 fiscal years ended December 31, 2013, 2014, and 2015, foreign exchange transaction gains and (losses) were included in other income (expense) and were $(1,196), $(3,188), and $975 respectively.

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. Valuation allowances are established when it is more likely than not that such deferred tax assets will not be realized.

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.

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 and potentially dilutive securities outstanding during the period. Potentially dilutive securities include stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock options 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. In 2013, 5,000,000 shares of common stock subject to the GE Warrant, as defined and described in notes 9 and 11, were included in the basic and dilutive 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 and diluted 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 basic and diluted net loss per share calculations, as 500,000 shares were exercisable in 2012 upon the execution of the associated Credit Agreement, as defined and described in note 9, 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 and as a result, 4,000,000 shares subject to the GE Warrant will not vest and will not become exercisable.

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



	2013	2014	2015
Stock options	11,526,998	11,486,301	11,487,938
Warrants	2,130,682	6,130,682	2,130,682
Convertibles notes	35,185,979	35,185,979	35,185,979
Restricted stock units	1,590,836	2,591,752	3,419,776

Comprehensive Income (Loss)

Comprehensive income (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 income (loss) for the years ended December 31, 2013, 2014, and 2015 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. However, certain international customers have historically been slower to pay on trade receivables. Accordingly, the Company continuously 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 $88,740 and $40,691 as of December 31, 2014 and 2015, respectively.

Recently Adopted Accounting Changes and Recently Issued and Adopted Accounting Standards

In February 2016, the FASB issued Accounting Standards Update ("ASU") No. 2016-02, Leases. The new standard

will require 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 fiscal 2019 and mandates a modified retrospective transition method. The Company is currently 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 fiscal 2018. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

In November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which amends existing guidance on income taxes to require the classification of all deferred tax assets and liabilities as non-current on the balance sheet. The standard is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods. Early adoption permitted and the guidance may be applied either prospectively or retrospectively. The Company has adopted this standard, which did not have a material impact on our financial statements, in our December 31, 2015 financial statements on a prospective basis.

In July 2015, the FASB issued ASU 2015-11, Simplifying the Measurement of Inventory, changing the measurement principle for inventory from the lower of cost or market to the lower of cost or net realizable value. Net realizable value is defined as the estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. The Company adopted this standard early to apply to the annual period ended December 31, 2015 and prospective annual periods, and it did not have a material impact on the Company’s financial statements for its fiscal year ended December 31, 2015.

In April 2015, the FASB issued ASU No. 2015-3, Interest - Imputation of Interest, requiring 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 updated guidance is effective retroactively for financial statements covering fiscal years beginning after December 15, 2015, and interim periods within those fiscal years, which for the Company is the first quarter of fiscal 2016. Early adoption was permitted but the Company did not elect early adoption. As of December 31, 2015, the Company has $4,023 of unamortized debt issuance costs.

In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, Deferral of the Effective Date.” With the issuance of ASU 2015-14, the new revenue guidance ASU 2014-09 will be effective for annual reporting periods beginning after 15 December 2017, including interim reporting periods within that reporting period, which for the Company is the first quarter of fiscal 2018, using one of two prescribed retrospective methods. The Company is currently in the process of evaluating the impact of adoption on its consolidated financial statements and related disclosures. The Company has not yet selected a transition method, nor has it determined the effect of the standard on its ongoing financial reporting.

In May 2014, the FASB issued ASU No. 2014-9, Revenue from Contracts with Customers, amending revenue recognition guidance and requiring more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

In August 2014, the FASB issued ASU No. 2014-15, to communicate amendments to FASB Accounting Standards Codification Subtopic 205-40, "Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern." The ASU requires management to evaluate relevant conditions, events and certain management plans that are known or reasonably knowable as of the evaluation date when determining whether substantial doubt about an entity's ability to continue as a going concern exists. Management will be required to make this evaluation for both annual and interim reporting periods. Management will need to make certain disclosures if it concludes that substantial doubt exists or when it plans to alleviate substantial doubt about the entity's ability to continue as a going concern. The standard is effective for annual periods ending after December 15, 2016 and for interim reporting periods starting in the first quarter of 2017. Early adoption is permitted. The Company does not expect the adoption of the guidance to have a material impact on its consolidated financial statements

Acquisitions and Divestitures

12 Months Ended

Dec. 31, 2015

Business Combinations [Abstract]

Acquisitions and Divestitures

BAF

On June 28, 2013, the Company, entered into and closed a stock purchase agreement (the "BAF Sale Agreement") with Westport Innovations Inc. ("Westport") and Westport Innovations (U.S.) Holdings Inc., a wholly owned subsidiary of Westport (together with Westport, the "Westport Parties"). Under the terms of the BAF Sale Agreement, on June 28, 2013, the Westport Parties purchased all of the outstanding capital stock of BAF Technologies, Inc. ("BAF"), including BAF's 100% ownership interest of ServoTech Engineering, Inc., for 816,460 shares of Westport's common stock. Pursuant to the BAF Sale Agreement, the Company was issued 718,485 shares of Westport's common stock on June 28, 2013 and 97,975 shares of Westport's common stock (the "Holdback Shares") were retained by Westport for one year as security for the Company's indemnification obligations under the BAF Sale Agreement. At the end of June 2014, the Company was issued 94,914 of the Holdback Shares, with the remaining 3,061 Holdback Shares remaining unissued as a result and in full satisfaction of an indemnity claim under the BAF Sale Agreement. In July 2013, the Company sold the 718,485 shares it received upon closing for net proceeds of $23,722. In July 2014, the Company sold all of the Holdback Shares it received for net proceeds of $1,727. Further, during August 2013, the Westport Parties repaid $2,478 of certain intercompany indebtedness of BAF to the Company following the conclusion of applicable post-closing adjustment procedures contemplated in the BAF Sale Agreement.

The fair value of the 816,460 shares of Westport's common stock on June 28, 2013 was $27,221, and the Company recognized an initial gain of $15,498 on June 28, 2013 related to the transaction. In December 2013, the Company wrote down the value of the Holdback Shares by $1,383, which resulted in an adjusted gain of $14,115 on the transaction. In July 2014, the Company wrote down the value of the Holdback Shares by an additional $122, which resulted in an adjusted gain of $13,993 on the transaction. The value of the shares of Westport's common stock received by the Company has been excluded from the Company's consolidated statements of cash flows as it is a non-cash investing activity. The gain was recorded in the line item gain from sale of subsidiary in the Company's consolidated statements of operations.

In addition, pursuant to the BAF Sale Agreement, the Company, Westport Power Inc. and Westport Fuel Systems Inc. (Westport Power, Inc. and Westport Fuel Systems, Inc. are collectively referred to as the "Westport Affiliates") entered into a marketing agreement, dated June 28, 2013, whereby the Westport Affiliates agreed to pay the Company $5,000 in cash, which was received on February 27, 2014. Under the marketing agreement, the Company and the Westport Affiliates agreed to collaborate during a two year period to encourage sales of all BAF products and certain vehicle products offered by the Westport Affiliates, and the Company agreed to provide 750,000 complimentary gasoline gallon equivalents of CNG to be used by the Westport Affiliates as marketing incentives. Additionally, the marketing agreement provides for the Company's appointment of a product line manager for BAF, and at least one member of a newly established operating committee formed to create sales and marketing strategies for BAF and assist in BAF's performance of these strategies.

MGES

On May 6, 2013, the Company entered into and closed a stock purchase agreement with Mansfield Energy Corp. ("Mansfield") and its wholly owned subsidiary Mansfield Gas Equipment Systems Corporation ("MGES"). MGES is primarily engaged in the business of providing CNG station design and construction and CNG equipment repair and maintenance services. Under the terms of the stock purchase agreement, the Company purchased from Mansfield all of the outstanding capital stock of MGES for $20,000, payable 50% in cash and 50% in shares of the Company's common stock. Upon closing, the Company delivered $9,000 in cash and 761,545 shares of the Company's common stock, and retained $1,000 in cash as security for Mansfield's indemnification obligations under the stock purchase agreement. On the first anniversary of the closing date, the Company delivered the retained amount of $1,000 to Mansfield. In addition, in August 2013, the Company paid Mansfield an additional $563 following the conclusion of applicable post-closing adjustment procedures contemplated by the stock purchase agreement. The fair value of the Company's common stock delivered to Mansfield is excluded from the Company's consolidated statements of cash flows as it is a non-cash investing activity. The Company filed with the Securities and Exchange Commission a registration statement covering the resale of such shares, and the registration statement was declared effective in August 2013.

The Company accounted for this acquisition in accordance with FASB authoritative guidance for business combinations, which requires the Company to recognize 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	$	4,475
Property, plant and equipment	1,369
Identifiable intangible assets	600
Goodwill	16,555
Total assets acquired	22,999
Current liabilities assumed	(1,984		)
Total purchase price	$	21,015

Management allocated approximately $600 of the purchase price to the identifiable intangible assets related to customer relationships and project back-orders 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 one to six 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 fully deductible for income tax purposes.

The results of operations of MGES have been included in the Company's consolidated financial statements since May 6, 2013. The historical results of MGES's operations were not material to the Company's financial position or historical results of operations.

NG Advantage

On October 14, 2014, the Company entered into a Common Unit Purchase Agreement ("UPA") with NG Advantage, LLC ("NG Advantage"). NG Advantage is engaged in the business of transporting CNG in high-capacity trailers to large industrial and institutional energy users, such as hospitals, food processors, manufacturers and paper mills, which 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 approximately $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 historical results of NG Advantage's operations were not material to the Company's financial position or historical results of operations.

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 Dallas Clean Energy McCommas Bluff, LLC ("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 the line item gain from the sale of subsidiary in the Company's statements of operations for 2014 and 2015, respectively. Included in the determination of the total gain is goodwill of $7,386 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, 2015

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 consisted of the following as of December 31, 2014 and 2015:



	December 31, 2014		December 31, 2015
Short-term restricted cash:
Standby letters of credit	$	1,753	$	1,631
Canton Bonds (see note 9)	4,259		2,609
Total short-term restricted cash	$	6,012	$	4,240

Investments

12 Months Ended

Dec. 31, 2015

Investments, Debt and Equity Securities [Abstract]

Investments

Available-for-sale investments 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 investments 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 investments 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, 2015, the Company believes its carrying values for its available-for-sale investments are properly recorded.

Short-term investments as of December 31, 2014 are summarized as follows:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	38,668	$	(16	)	$	38,652
Zero coupon bonds	3,308		(2		)	3,306
Corporate bonds	45,274		(41		)	45,233
Certificate of deposits	35,355		—			35,355
Total short-term investments	$	122,605	$	(59	)	$	122,546

Short-term investments as of December 31, 2015 are summarized as follows:



	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

- Definition

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

+ References + Details

Other Receivables

12 Months Ended

Dec. 31, 2015

Receivables [Abstract]

Other Receivables

Other receivables at December 31, 2014 and 2015 consisted of the following:



	2014		2015
Loans to customers to finance vehicle purchases	$	8,257	$	10,531
Accrued customer billings	10,143		7,106
Fuel tax and carbon credits	34,250		40,730
Other	3,573		2,300
	$	56,223	$	60,667

Land, Property and Equipment

12 Months Ended

Dec. 31, 2015

Property, Plant and Equipment [Abstract]

Land, Property and Equipment

Land, property and equipment at December 31, 2014 and 2015 are summarized as follows:



	2014			2015
Land	$	2,858		$	2,858
LNG liquefaction plants	94,636			94,634
RNG plants	45,359			46,397
Station equipment	265,086			316,258
LNG trailers	40,067			50,414
Other equipment	74,796			83,687
Construction in progress	163,737			139,586
	686,539			733,834
Less accumulated depreciation	(172,270		)	(217,510		)
	$	514,269		$	516,324

Included in the land, property and equipment are capitalized software costs of $21,004 and $22,886 as of December 31, 2014 and December 31, 2015, respectively. The accumulated amortization on the capitalized software costs is $10,740 and $13,793 as of December 31, 2014 and December 31, 2015, respectively. The Company recorded $3,079, $2,993, and $3,053 of amortization expense related to the capitalized software costs in 2013, 2014 and 2015, respectively.

As of December 31, 2014 and 2015, $11,032 and $5,955, respectively, are included in accounts payable balances, which amounts are related to purchases of property and equipment within the respective year. 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, 2015
Investments, All Other Investments [Abstract]
Investment in Other Entities	Investment in Other Entities The Company previously invested in Clean Energy del Peru ("Peru JV"), a joint venture in Peru that operated CNG stations. The Company accounted for its investment in Peru JV under the equity method of accounting as the Company had the ability to exercise significant influence over Peru JV's operations while the Company maintained an ownership interest in Peru JV. In March 2013, the Company completed the sale of its entire ownership interest in Peru JV for $6,119 after receiving a dividend distribution of $1,091, and recognized a gain of $4,705. 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 U.S. The Company and Mansfield Ventures each have a 50% ownership interest in MCEP. The Company accounts for its interest 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 and $815 has an investment balance of $5,510 and $4,695 at December 31, 2014 and December 31, 2015, respectively.

Accrued Liabilities

12 Months Ended

Dec. 31, 2015

Payables and Accruals [Abstract]

Accrued Liabilities

Accrued liabilities at December 31, 2014 and 2015 consisted of the following:



	2014		2015
Salaries and wages	$	9,041	$	9,537
Accrued gas and equipment purchases	12,340		14,133
Accrued property and other taxes	5,178		5,344
Accrued professional fees	1,084		1,105
Accrued employee benefits	3,208		3,042
Accrued warranty liability	2,302		1,826
Accrued interest	3,748		3,718
Other	19,859		20,377
	$	56,760	$	59,082

Debt

12 Months Ended

Dec. 31, 2015

Debt Disclosure [Abstract]

Debt

Debt and capital lease obligations at December 31, 2014 and 2015 consisted of the following and are further discussed below:



	December 31, 2014			December 31, 2015
7.5% Notes(1)	$	150,000		$	150,000
SLG Notes	145,000			145,000
5.25% Notes	250,000			250,000
Canton Bonds	12,150			10,910
Capital lease obligations	2,692			6,448
Other debt	10,828			10,056
Total debt and capital lease obligations	570,670			572,414
Less amounts due within one year	(4,846		)	(150,129		)
Total long-term debt and capital lease obligations	$	565,824		$	422,285

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

The following is a summary of aggregate maturities of long-term debt and capital lease obligations for each of the years ending December 31:



	2016		2017		2018		2019		2020		Thereafter
7.5% Notes(1)	—		—		50,000		50,000		50,000		—
SLG Notes	145,000		—		—		—		—		—
5.25% Notes	—		—		250,000		—		—		—
Canton Bonds	1,390		1,420		1,460		1,555		1,665		3,420
Capital lease obligations	1,294		1,329		1,427		1,425		440		533
Other debt	2,445		2,706		2,227		1,671		1,007		—
Total	$	150,129	$	5,455	$	305,114	$	54,651	$	53,112	$	3,953

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"), Boone 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 ownership 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 "June 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 June Advance (the replacement notes and the notes issued in exchange for the June 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, the holders of the 7.5% Notes have the right to exchange all or any portion of the principal and accrued and unpaid interest under each such note 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 original issuance date and the Company may repay each 7.5% Note in shares of its common stock 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 restrict the use of the proceeds of the 7.5% Notes to financing the development, construction and operation of LNG stations and payment of certain related expenses. The Amended Agreements also 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. At December 31, 2015, none of the proceeds from the 7.5% Notes were included in restricted cash as the Company had used the funds primarily to build LNG fueling stations. No events of default under the 7.5% Notes had occurred as of December 31, 2015.

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, 2015, (i) Boone Pickens holds 7.5% Notes in the aggregate principal amount of $65,000, which 7.5% Notes are convertible into approximately 4,113,924 shares of the Company's common stock, (ii) Green Energy Investment Holdings, LLC holds 7.5% Notes in the aggregate principal amount of $80,000, which 7.5% Notes are convertible into approximately 5,063,291 shares of the Company's common stock, and (iii) other third parties hold 7.5% Notes in the aggregate principal amount of $5,000, which 7.5% Notes are convertible into approximately 316,456 shares of the Company's common stock.

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"), whereby 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.

The SLG Notes bear interest at the rate of 7.5% per annum and are convertible at the option of each Purchaser into shares of the Company's common stock at a conversion price of $15.00 per share (the "SLG Conversion Price"). Upon written notice to the Company, the holders of the SLG Notes have the right to exchange all or any portion of the principal and accrued and unpaid interest under each such note for shares of the Company's common stock at the SLG Conversion Price. Additionally, subject to certain restrictions, the Company can force conversion of each SLG Note into shares of its common stock if, following the second anniversary of the issuance of the SLG Notes, such shares trade at a 40% premium to the SLG Conversion Price for at least 20 trading days in any consecutive 30 trading day period. The entire principal balance of each SLG Note is due and payable five years following its issuance and the Company may repay the principal balance of each SLG Note in shares of its common stock or cash. All of the shares issuable upon conversion of the SLG 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 SLG Agreements include certain affirmative and negative covenants relating to the Company’s operations, including, among others, a prohibition on the Company’s declaration or payment of any dividends or distributions with respect to its capital stock, except for those payable solely with the Company’s common stock, without the prior consent of a majority of the holders of the outstanding SLG Notes. The SLG Agreements also provide for customary events of default which, if any of them occurs, would permit or require the principal of, and accrued interest on, the SLG Notes to become, or to be declared, due and payable. No events of default under the SLG Notes had occurred as of December 31, 2015.

In April 2012, $1,003 of principal and accrued interest under an SLG Note was converted by the holder thereof into 66,888 shares of the Company's common stock. In January and February 2013, $4,030 of principal and accrued interest under an SLG Note was converted by the holder thereof into 268,664 shares of the Company's common stock. Such conversions were not included in the consolidated statements of cash flows as they are a non-cash financing activity.

On February 29, 2016, the Company repaid $60,000 in cash of the $145,000 outstanding principal amount of the SLG Notes.

5.25% Notes

In September 2013, the Company completed a private offering 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, and intends to continue to use, 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 had occurred under the 5.25% Notes as of December 31, 2015.

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.

On February 18, 2016, the Company paid $16,761 in cash to retire $32,500 par value and accrued interest of the $250,000 of the 5.25% Notes.

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 the Borrower'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 accelerate all amounts due under the Loan Agreement. No events of default had occurred as of December 31, 2015.

Other Debt

The Company has other debt due at various dates through 2020 bearing interest at rates up to 19.33% and with a weighted average interest rate of 6.80% and 6.35% as of December 31, 2014 and 2015, 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, 2013 and 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 11 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 thirty 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 related documents but excluding the GE Warrant which remains outstanding, although 4,000,000 shares subject to the GE Warrant will not vest and will not become exercisable; see note 11 for further information. 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 $54.9 million 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 Maxrix 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.

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”). All amounts advanced under the Credit Facility are due and payable on February 29, 2017. Simultaneously, the Company drew down $50,000 under this Credit Facility. The Credit Facility is evidenced by a promissory note we 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 our 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 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.

Derivative Transactions	12 Months Ended
Derivative Transactions	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Transactions	Derivative Transactions The Company had no commodity futures contracts or forward exchange contracts outstanding during 2015. 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. In the years ended December 31, 2013, and 2014, the Company recorded unrealized gains of $2,151, and $108, respectively, in other comprehensive income (loss) related to its futures contracts. Of the Company's net futures contracts liability of $107 at December 31, 2013, $5 was recorded as an asset in prepaid expenses and other current assets and $112 was recorded as an accrued liability in the Company's consolidated balance sheet at December 31, 2013. The Company's ineffectiveness related to its futures contracts and forward exchange contracts for the year ended December 31, 2013 was insignificant. During the years ended December 31, 2013 and 2014, the Company recognized a loss of $2,370 and $65, respectively, in cost of sales in the accompanying consolidated statements of operations related to its futures contracts that were settled during the respective years. These amounts were 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

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+ Details

Stockholders' Equity

12 Months Ended

Dec. 31, 2015

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, 2015, 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, 2013, 2014 or 2015.

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

Unit Sale and Issuance

On October 28, 2008, the Company entered into a placement agent agreement (the "Placement Agent Agreement") relating to the sale and issuance by the Company to select investors of 4,419,192 units (the "Units"), with each Unit consisting of (i) one share of the Company's common stock, (ii) a warrant to purchase up to 0.75 shares of the Company's common stock (the "Series I Warrant"), and (iii) one warrant to purchase up to 0.2571 shares of the Company's common stock (the "Series II Warrant"). The price of each Unit was $7.92 per Unit. The transaction closed on November 3, 2008, and the Company issued 4,419,192 shares of common stock, Series I Warrants to purchase up to 3,314,394 shares of common stock, and Series II Warrants to purchase up to 1,136,364 shares of common stock. The Company received approximately $32,484 after deducting the placement agent's fees and other offering expenses related to the Unit sale. The Company allocated $19,166, $9,745 and $3,573 of the proceeds to the common stock, the Series I Warrants and the Series II Warrants, respectively.

The Series I Warrants became exercisable beginning six months from the date of issuance, have a term of seven years from the date they became exercisable, and carry an exercise price of $12.54 per share. On November 10, 2010, the Company entered into an amendment with one of the holders of the Series I Warrants pursuant to which the expiration date of such warrant for the purchase of 1,183,712 shares of common stock was changed to November 10, 2010. In consideration of the modification to the expiration date, the Company agreed to pay the holder of such warrant approximately $3,172. The Company received notice on November 10, 2010 that such warrant was being exercised in full, and issued 1,183,712 shares of its common stock for an aggregate exercise price of approximately $15,009. Upon exercise, the Company recognized a gain of approximately $3,208 related to the transaction. For additional information on the Series I Warrants, see note 18.

The Series II Warrants became exercisable on November 5, 2008 upon the failure of the California Alternative Fuel Vehicles and Renewable Energy Act, or Proposition 10, in the California statewide election. The Series II Warrants were all exercised on a cashless basis at the exercise price of $0.01 per share, which resulted in the issuance of 1,134,759 shares of common stock to the Series II Warrant holders on November 12, 2008.

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. At issuance, 500,000 shares were immediately exercisable and an additional 500,000 shares became exercisable on December 31, 2014. The Company terminated the Credit Agreement on December 31, 2015 and as a result, the remaining 4,000,000 shares subject to the GE Warrant will not vest and will not become exercisable.

The GE Warrant terminates on November 7, 2022. During the exercise period, if the Company issues or sells any shares of its common stock other than exempted securities (as defined in the GE Warrant) for a price per share less than a price equal to 80% of the market price on the day of such issuance or sale, then, immediately after such issuance and sale, the number of shares then purchasable shall be increased on a proportionate basis by a formula set forth in the GE Warrant. All of the shares issuable upon exercise of the GE Warrant have been registered for resale by the holder thereof pursuant to a registration statement filed with and declared effective by the Securities and Exchange Commission.

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 500,000 shares that were immediately exercisable and the 500,000 shares that became exercisable on December 31, 2014 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 balance sheet in full through a non-cash charge to earnings, reported in interest expense, of $54,925 in the fourth quarter of 2015. See note 9 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 its common stock having an aggregate offering price of up to $75,000 in an “at-the-market” offering program (the “ATM Program”). As of December 31, 2015, the Company received $6,450 in proceeds, net of $493 in fees and issuance costs, and issued 1,561,902 shares of its common stock in the ATM Program.

Other

At December 31, 2015, third parties held outstanding warrants, which expire in 2020, to purchase equity interests in NG Advantage. Such warrants allow a purchase up to 127,200 shares of NG Advantage common units and are treated as liability-classified warrants. The fair value was $630 and $561 as of December 31, 2014 and 2015, respectively and the gain from the change in fair value was $0 and $69 for 2014 and 2015, respectively. See note 18.

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,
	2013		2014		2015
Stock-based compensation expense, net of $0 tax in 2013, 2014 and 2015	$	23,008	$	11,514	$	10,779

Equity Incentive Plans

In December 2002, the Company adopted its 2002 Stock Option Plan ("2002 Plan"). When the 2002 Plan was available for the issuance of new awards, the Company's board of directors determined eligibility, vesting schedules, and exercise prices for options granted thereunder. Options generally have a term of ten years.

Under the 2002 Plan, eligible persons could be issued options for services rendered to the Company. Under the 2002 Plan, the purchase price per share for each option granted could not be less than 100% of the fair market value of the Company's common stock on the date of such option grant; provided, however, that the purchase price per share of common stock issued to a 10% stockholder could not be less than 110% of the fair market value of the Company's common stock on the date of such option grant. Options generally vest over a three-year period.

In December 2006, the Company adopted its 2006 Equity Incentive Plan ("2006 Plan"). The 2006 Plan was 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. As of December 31, 2015, the Company had 19,890,500 shares approved for issuance under its option plans. At December 31, 2015, the Company had 262,461 shares available for future grant under the 2006 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 at December 31, 2012	12,083,677		$	11.75
Granted	98,500		13.47
Exercised	(119,349	)	5.24
Forfeited or Expired	(535,830	)	12.64
Options Outstanding at 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 at 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 at December 31, 2015	11,487,938		$	11.44	4.93	—
Options Exercisable at December 31, 2015	9,736,608		$	12.32	4.13	—

As of December 31, 2015, there was $4,907 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 2.28 years. The total fair value of shares vested during the year ended December 31, 2015 was $7,958.

The intrinsic value of all stock options exercised during 2013, 2014 and 2015 was $935, $2,568 and $2,197 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,
	2013	2014	2015
Dividend yield	0.0%	0.0%	0.0%
Expected volatility	51.0% to 55.6%	52.3% to 67.0%	59.2% to 72.0%
Risk-free interest rate	1.0% to 1.9%	1.1% to 1.8%	1.7% to 1.8%
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, 2013, 2014 and 2015, were $6.86, $5.32 and $3.29, respectively. The volatility amounts used during the periods were estimated based on the Company's historical volatility and the Company's implied volatility of its traded options. The expected lives used during the periods were based on historical exercise periods and the Company's anticipated exercise periods for its outstanding stock options. The risk free interest rates used during the periods 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 $13,751, $7,286 and $5,195 of stock option expense during the years ended December 31, 2013, 2014 and 2015, 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, on January 25, 2016, Market-Based RSUs granted in January 2012 and entitling the holders to receive 1,215,000 shares of the Company’s common stock were 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 at December 31, 2012	1,545,000		$	11.42
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding at December 31, 2013	1,545,000		$	11.42
Granted	489,500		8.26
Vested	—		—
Forfeited or Expired	(265,500	)	10.62
RSU Outstanding at December 31, 2014	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding and Unvested at December 31, 2015	1,769,000		$	10.67	0.58

As of December 31, 2015, there was $2,063 of total unrecognized compensation cost related to unvested shares underlying outstanding Market-Based RSUs. That cost is expected to be expensed over a remaining weighted average period of 0.09 year.

The Company recorded $8,821, $2,556 and $1,770 of expense in 2013, 2014 and 2015, 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 Plan 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 at December 31, 2012	—		$	—
Granted	45,836		13.09
Vested	—		—
Forfeited or expired	—		—
RSU Outstanding at December 31, 2013	45,836		$	13.09
Granted	792,500		5.54
Vested	(15,584	)	13.09
Forfeited or expired	—		—
RSU Outstanding at 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 and Unvested at December 31, 2015	1,650,776		$	5.50	2.19

As of December 31, 2015, there was $7,535 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 2.25 years.

The Company recorded $51, $365 and $2,622 of expense in 2013, 2014 and 2015, 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, 2015 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 $29, $67 and $50 of expense related to the ESPP during 2013, 2014 and 2015, respectively. The Company has not recorded any tax benefits related to its ESPP expense. As of December 31, 2015, the Company had issued an aggregate of 152,389 shares pursuant to the ESPP.

Non-Qualified Non-Public Subsidiary Unit Options

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

The following table summarizes Clean Energy Renewables' unit option activity during the year ended December 31, 2015:



	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding at December 31, 2012	—		$	—
Options granted	115,000		40.80
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding at December 31, 2013	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding at December 31, 2014	115,000		$	40.80
Options granted	—		—		—	—
Options exercised	—		—
Options forfeited or expired	(7,000	)	40.80
Options Outstanding and non-vested, December 31, 2015	108,000		$	40.80		$	—

As of December 31, 2015, there was $803 of total unrecognized compensation cost related to unvested units underlying outstanding unit options. That cost is expected to be expensed over a remaining period of 0.50 years.

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



Dividend yield	0.0	%
Expected volatility	96.4	%
Risk-free interest rate	1.9	%
Expected life in years	6.0

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 amounts used during the period were estimated based on the historical volatility of a certain peer group of Clean Energy Renewables for a period commensurate with the expected life of the unit options granted. The expected life used was Clean Energy Renewables' anticipated exercise periods for its outstanding unit options. The risk free interest rate was based on the U.S. Treasury yield curve for the expected life of the unit options at the time of grant. Clean Energy Renewables recorded $356, $1,240 and $1,115 of unit option expense during 2013, 2014 and 2015, respectively. Clean Energy Renewables has not recorded any tax benefit related to its unit option expense.

Income Taxes

12 Months Ended

Dec. 31, 2015

Income Tax Disclosure [Abstract]

Income Taxes

The components of loss before income taxes for the years ended December 31, 2013, 2014 and 2015 are as follows:



	2013			2014			2015
U.S.	$	(40,195	)	$	(64,913	)	$	(111,437	)
Foreign	(23,009		)	(24,871		)	(22,407		)
	$	(63,204	)	$	(89,784	)	$	(133,844	)

The provision for income taxes consists of the following:



	2013		2014			2015
Current:
Federal	$	87	$	190		$	9
State	310		238			248
Foreign	2,910		1,017			912
Total current	3,307		1,445			1,169
Deferred:
Federal	134		29			337
State	47		(10		)	71
Foreign	227		(389		)	37
Total deferred	408		(370		)	445
Total	$	3,715	$	1,075		$	1,614

Income tax expense for the years ended December 31, 2013, 2014 and 2015 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following:



	2013			2014			2015
Computed expected tax (benefit)	$	(22,121	)	$	(30,415	)	$	(46,846	)
Nondeductible expenses	7,216			10,690			24,998
Tax rate differential on foreign earnings	2,993			5,733			3,701
Basis difference on sale	(6,457		)	—			—
Tax credits	(35,604		)	(8,286		)	(9,988		)
Other	82			(1,121		)	(372		)
Change in valuation allowance	57,606			24,474			30,121
Total tax expense	$	3,715		$	1,075		$	1,614

During 2013, federal tax legislation extended the VETC through December 31, 2013 with retroactive effect to the beginning of 2012, during 2014, federal tax legislation extended the VETC through December 31, 2014 with retroactive effect to the beginning of 2014 and during 2015, federal tax legislation extended the VETC through December 31, 2016 with retroactive effect to the beginning of 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 $9,298, $8,221 and $7,068 related to the exclusion of VETC associated with 2015 2014 and 2013 fuel sales in excess of its fuel tax obligation and a federal tax benefit of $27,497 was recorded in 2013 related to the exclusion of similar VETC amounts associated with fuel sales from 2006 through 2012. 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, 2014 and 2015 are as follows:



	2014			2015
Deferred tax assets:
Accrued expenses	$	4,834		$	4,512
Sales-type leases	908			154
Alternative minimum tax and general business credits	4,966			5,780
Derivative loss	527			—
Stock option expense	21,741			23,113
Other	2,596			2,283
Loss carryforwards	147,706			174,157
Total deferred tax assets	183,278			209,999
Less valuation allowance	(160,436		)	(189,203		)
Net deferred tax assets	22,842			20,796
Deferred tax liabilities:
Depreciation and amortization	(20,028		)	(17,398		)
Goodwill	(3,864		)	(4,600		)
Partnership income	—			(293		)
Total deferred tax liabilities	(23,892		)	(22,291		)
Net deferred tax liabilities	$	(1,050	)	$	(1,495	)

At December 31, 2015, the Company had federal, state and foreign net operating loss carryforwards of approximately $438,384, $336,251 and $58,090, respectively. The Company's federal, state and foreign net operating loss carryforwards will, if not utilized, expire beginning in 2026, 2016 and 2028, respectively. The Company also has federal tax credit carryforwards of $5,572 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, 2014 and 2015, the Company provided a valuation allowance of $160,436, and $189,203, 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, 2014 and 2015 was $23,468, and $28,767, 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, 2015, the Company has not provided deferred U.S. income taxes or foreign withholding taxes on temporary differences of approximately $2,828 resulting from earnings of certain non-U.S. subsidiaries which are permanently reinvested outside the U.S. 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 $27,497 at December 31, 2015 included $692 of tax benefits that, if recognized, would reduce the Company's annual effective tax rate. The remaining $26,805, 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, 2013, 2014 and 2015:



Unrecognized tax benefit—December 31, 2013	$	17,398
Gross increases—tax positions in current year	4,722
Gross increases—tax positions in prior years	(146		)
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

The increase in the Company's unrecognized tax benefits during 2014 and 2015 is primarily attributable to the portion of VETC offset by the fuel tax the Company collected from its customers as an unrecognized tax benefit during 2014 and 2015. The Company believes the portion of VETC 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.

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 $120 and $178 of interest expense and penalties as of December 31, 2014 and 2015, respectively. The Company recognized interest expense related to uncertain tax positions of $48, $54 and $58 during 2013, 2014 and 2015, respectively.

The Company is subject to taxation in the United States and various states and foreign jurisdictions. The Company's tax years for 2011 through 2015 are subject to examination by various tax authorities. The Company is no longer subject to U.S. examination for years before 2012, and for state tax examinations for years before 2011.

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 any particular 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. Other than an increase attributable to the VETC received in 2016 that offsets fuel tax liability, which cannot be accurately estimated, 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, 2015

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 that arise in the ordinary course of its business. During the course of its operations, the Company is also subject to audit by tax authorities for varying periods in various federal, state, local and foreign tax jurisdictions. Disputes may arise during the course of such audits as to facts and matters of law. It is impossible to determine the ultimate liabilities that the Company may incur resulting from any of these lawsuits, claims 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, an outcome not currently anticipated, 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. However, the Company believes that the ultimate resolution of such 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, 2015:



Fiscal year:
2016	$	7,998
2017	7,750
2018	6,106
2019	5,909
2020	5,388
Thereafter	20,874
Total future minimum lease payments	$	54,025

Rent expense, including variable rent, totaled $10,504, $10,140, and $8,629 for the years ended December 31, 2013, 2014 and 2015, 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, on a take-or-pay basis, 45,000 gallons of LNG per day, which was increased to 65,000 gallons per day in March 2014, from a plant constructed by DGS in Ehrenberg, Arizona, which is near the California border. This obligation began in March 2010, and for the years ended December 31, 2013, 2014 and 2015, the Company paid approximately $11,404, $14,267, and $11,852 respectively, under this contract. The contract expires in October 2017. At December 31, 2015, the fixed commitments under this contract totaled approximately $4,579 and $3,816 for the years ending December 31, 2016 and 2017, 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 of 2016 and extended to expire in December 2020. At December 31, 2015, the fixed commitments under the JTA contract totaled approximately $71, $203, $313, $429 and $548 for the years ending December 31, 2016, 2017, 2018, 2019, 2020, respectively.

Geographic Information

12 Months Ended

Dec. 31, 2015

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 disaggregated information about revenues 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 looking at the economics of a mix of product and services for a customer.

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 which 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) is based on the location of the entity selling the finished goods or providing the services.



	2013			2014			2015
Revenue:
United States	$	292,250		$	360,881		$	330,003
Canada	13,922			16,241			21,818
Other	46,303			51,818			32,499
Total revenue	$	352,475		$	428,940		$	384,320
Operating income (loss):
United States	$	(44,797	)	$	(41,543	)	$	(33,067	)
Canada	(1,594		)	(3,087		)	(4,980		)
Other	(5,300		)	(9,734		)	(3,576		)
Total operating income (loss)	$	(51,691	)	$	(54,364	)	$	(41,623	)
Long-lived assets:
United States	$	544,638		$	582,028		$	582,644
Canada	103,997			85,984			68,292
Other	7,537			6,854			5,693
Total long-lived assets	$	656,172		$	674,866		$	656,629

The Company's goodwill and intangible assets at December 31, 2013, 2014 and 2015 relate to its United States operations and its BAF (through June 28, 2013, see note 2), Clean Energy Compression, Clean Energy Cryogenics, and NG Advantage (beginning on October 14, 2014, see note 2) operations.

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=51669610&loc=d3e8981-108599

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-URI http://asc.fasb.org/extlink&oid=51669610&loc=d3e9038-108599

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 33
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+ Details

401(k) Plan	12 Months Ended
401(k) Plan	Dec. 31, 2015
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 make discretionary contributions to the Savings Plans that are subject to limitations. For the years ended December 31, 2013, 2014 and 2015 the Company contributed approximately $1,448, $1,040, and $304 of matching contributions to the Savings Plan, respectively.

Supplier Concentrations	12 Months Ended
Supplier Concentrations	Dec. 31, 2015
Risks and Uncertainties [Abstract]
Supplier Concentrations	Supplier Concentrations During 2013, 2014 and 2015, the Company incurred approximately 3%, 4%, and 1% respectively, of its natural gas expense related to its LNG sales from Williams Gas Processing Company pursuant to a floating rate purchase contract that includes minimum purchase commitments. During 2013, 2014 and 2015, the Company incurred approximately 22%, 17%, and 16% respectively, of its natural gas expense related to its LNG sales from Shell Energy, which supplied the Company's LNG plant in California and supplies DGS's plant in Arizona, where the Company has a take or pay obligation to DGS. See note 13 for further information regarding the DGS take or pay obligation. During 2013, 2014 and 2015, the Company incurred approximately 7%, 3%, and 7%, respectively, of its natural gas costs related to its CNG operations from the SoCal Gas Company and San Diego Gas and Electric. The Company incurred approximately 24% and 19% of its natural gas expense related to its LNG and CNG purchases from BP Energy during 2014 and 2015, respectively. Vermont Gas Systems supplies natural gas to our subsidiary NG Advantage. For the years ended 2014 and 2015 Vermont Gas Systems accounted for approximately 0% and 10% of the Company's natural gas expense, respectively.

Capitalized Lease Obligation and Receivables

12 Months Ended

Dec. 31, 2015

Leases [Abstract]

Capitalized Lease Obligation and Receivables

The Company leases equipment under capital leases with a weighted-average interest rate of 9.71%. At December 31, 2015, future payments under these capital leases are as follows:



2016	$	1,688
2017	1,738
2018	1,739
2019	1,633
2020	660
Thereafter	633
Total minimum lease payments	8,091
Less amount representing interest	(1,643		)
Future minimum lease payments	6,448
Less current portion	(1,294		)
Capital lease obligations, less current portion	$	5,154

The value of the equipment under capital lease as of December 31, 2014 and 2015 was $4,898 and $8,970, with related accumulated amortization of $1,936 and $2,766, respectively.

The Company also leases certain fueling station equipment to certain customers under sales-type leases at a weighted average effective interest rate of 10.62%. The leases are payable in varying monthly installments through February 2018.

At December 31, 2015, future receipts under these leases are as follows:



2016	$	563
2017	380
2018	251
2019	186
2020	186
Thereafter	1,798
Total	3,364
Less amount representing interest	(1,575		)
	$	1,789

Fair Value Measurements

12 Months Ended

Dec. 31, 2015

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 nonrecurring 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.

During the twelve months ended December 31, 2015, the Company's financial instruments consisted of available-for-sale securities, debt instruments and liability-classified warrants (some of which are our Series I 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 at December 31, 2014 and 2015.

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, 2014 and December 31, 2015, respectively:



Description	Balance at December 31, 2014		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Certificate of deposits	$	35,355	$	—	$	35,355	$	—
Municipal bonds and notes	38,652		—		38,652		—
Zero coupon bonds	3,306		—		3,306		—
Corporate bonds	45,233		—		45,233		—
Liabilities:
Warrants(2)	1,416		—		—		1,416



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

_______________________________________________________________________________


(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	2014			2015
Beginning Balance	$	7,164		$	2,046
Gain included in earnings	(5,748		)	(1,414		)
Warrants acquired from NG Advantage acquisition (see note 11)	630			—
Ending Balance	$	2,046		$	632

Valuation Processes for Level 3 Fair Value Measurements and Sensitivity to Changes in Significant Unobservable Inputs

The fair value measurements are compared to those of the prior reporting periods to ensure that changes are consistent with expectations of management based upon the sensitivity and nature of the inputs.

Series I Warrant Liability

The Company estimated the fair value of its Series I Warrant liability using the Black-Scholes option pricing model based on the following inputs as of December 31, 2015:



Unobservable Inputs	Range or Weighted Average
Current market price of the Company's common stock	$	3.60
Exercise price of the warrant	$	12.54
Dividend yield	0.00		%
Remaining term of the warrant	0.33
Implied volatility of the Company's common stock	83.05		%
Assumed discount rate	0.33		%

Significant changes in any of those inputs in isolation can result in a significant change in the fair value measurement. Generally, a positive change in the market price of the Company's common stock, an increase in the volatility of the Company's common stock, or an increase in the remaining term of the warrant would result in a directionally similar change in the estimated fair value of the Company's Series I Warrants and thus an increase in the associated liability. An increase in the assumed discount rate or a decrease in the positive differential between the warrant's exercise price and the market price of the Company's common stock would result in a decrease in the estimated fair value measurement of the Series I Warrants and thus a decrease in the associated liability. The Company has not, nor does it plan to, declare dividends on its common stock, and thus, there is no directionally similar change in the estimated fair value of the warrants due to the dividend assumption.

- 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

Schedule II: Valuation and Qualifying Accounts

12 Months Ended

Dec. 31, 2015

Valuation and Qualifying Accounts [Abstract]

Schedule II: Valuation and Qualifying Accounts



	Allowances for Doubtful Trade Receivables			Allowance for Doubtful Notes Receivables
Balance at December 31, 2012	$	905		$	909
Charges (benefit) to operations	454			1,507
Deductions	(527		)	—
Balance at December 31, 2013	832			2,416
Charges (benefit) to operations	387			890
Deductions	(467		)	(456		)
Balance at December 31, 2014	752			2,850
Charges (benefit) to operations	1,514			1,142
Deductions	(371		)	(2		)
Balance at December 31, 2015	$	1,895		$	3,990

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to state fairly the Company's financial position, results of operations, comprehensive income (loss) and cash flows in accordance with U.S. generally accepted accounting principles ("US GAAP"). All intercompany accounts and transactions have been eliminated in consolidation.
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 reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and revenues and expenses recorded during the reporting periods. Actual results could differ from those estimates. 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, short-term investments, accounts and other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable approximate fair value.
Inventories	Inventories 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 includes 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.
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 has determined that it is a single reporting unit for the purpose of goodwill impairment tests. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review.
Revenue Recognition	The Company’s volume related revenues primarily consist of CNG and LNG fuel sales, RIN and LCFS Credits sales and O&M services. These revenues are recognized in accordance with US GAAP, which requires that the following four criteria must be met before revenue can be recognized: (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 it generates RIN Credits when it sells RNG as a vehicle fuel in the U.S. 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 at the time it has an agreement in place to sell the credits at a fixed or determinable price. The Company recognizes compression revenues through its subsidiary Clean Energy Compression when it sells non-lubricated natural gas fueling compressors and related 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. The Company typically recognizes revenue on fueling station construction projects where it sells the station or station upgrade to the customer using the completed-contract method because the projects are short-term and the results of operations reported on the completed-contract basis would not vary materially from those resulting from the use of the percentage-of-completion method. The construction contract is considered to be substantially completed at the earlier of customer acceptance of the fueling station or the time when the fuel dispensing activities begin. When applicable, multi-station construction contracts are segmented into phases as negotiated with customers. Gross margin related to each phase is recognized when it is substantially complete. The Company’s subsidiary Clean Energy Cryogenics, which manufactures components used in the Company’s station sales, 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. In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction 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 completed-contract 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. 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. The Company is eligible to receive, at times, a federal alternative fuels tax credit ("VETC") when a gasoline gallon equivalent of CNG and a liquid gallon of LNG is sold as vehicle fuel. 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 refundable and do not need to offset income tax liabilities to be received. See the discussion under “Alternative Fuels Excise Tax Credit” below for further information. The Company previously owned BAF Technologies, Inc. and its wholly owned subsidiary, ServoTech Engineering, Inc. (BAF Technologies, Inc. and ServoTech Engineering Inc. are collectively referred to as "BAF"), which converted light and medium duty vehicles to run on natural gas and provided design and engineering services for natural gas engine systems. On June 28, 2013, the Company sold BAF to Westport Innovations (U.S.) Holdings Inc., a wholly owned subsidiary of Westport Innovations Inc. As of the sale of BAF, the Company no longer generates revenues from vehicle conversions. Revenue Recognition The Company recognizes revenue on various products and services.
Alternative Fuels Excise Tax Credit	Alternative Fuels Excise Tax Credit From October 1, 2006 through December 31, 2015, the Company was eligible to receive the VETC alternative fuels tax credit of $0.50 per gasoline gallon equivalent of CNG and $0.50 per liquid gallon of LNG that it sold as vehicle fuel. For 2016, the VETC credit is $0.50 per gasoline gallon equivalent of CNG and $0.50 per diesel gallon equivalent of LNG that is sold as a vehicle fuel. The American Taxpayer Relief Act, signed into law on January 2, 2013, reinstated VETC for 2013 and made it retroactive to January 1, 2012.
LNG Transportation Costs	LNG Transportation Costs The Company records the costs incurred to transport LNG to its customers in the line item cost of sales in the accompanying 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.
Foreign Currency Translation	Foreign Currency Translation In accordance with FASB authoritative guidance, 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. Net 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.
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. Valuation allowances are established when it is more likely than not that such deferred tax assets will not be realized. 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.
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 and potentially dilutive securities outstanding during the period. Potentially dilutive securities include stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock options 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.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (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. However, certain international customers have historically been slower to pay on trade receivables. Accordingly, the Company continuously 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 February 2016, the FASB issued Accounting Standards Update ("ASU") No. 2016-02, Leases. The new standard will require 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 fiscal 2019 and mandates a modified retrospective transition method. The Company is currently 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 fiscal 2018. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which amends existing guidance on income taxes to require the classification of all deferred tax assets and liabilities as non-current on the balance sheet. The standard is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods. Early adoption permitted and the guidance may be applied either prospectively or retrospectively. The Company has adopted this standard, which did not have a material impact on our financial statements, in our December 31, 2015 financial statements on a prospective basis. In July 2015, the FASB issued ASU 2015-11, Simplifying the Measurement of Inventory, changing the measurement principle for inventory from the lower of cost or market to the lower of cost or net realizable value. Net realizable value is defined as the estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. The Company adopted this standard early to apply to the annual period ended December 31, 2015 and prospective annual periods, and it did not have a material impact on the Company’s financial statements for its fiscal year ended December 31, 2015. In April 2015, the FASB issued ASU No. 2015-3, Interest - Imputation of Interest, requiring 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 updated guidance is effective retroactively for financial statements covering fiscal years beginning after December 15, 2015, and interim periods within those fiscal years, which for the Company is the first quarter of fiscal 2016. Early adoption was permitted but the Company did not elect early adoption. As of December 31, 2015, the Company has $4,023 of unamortized debt issuance costs. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, Deferral of the Effective Date.” With the issuance of ASU 2015-14, the new revenue guidance ASU 2014-09 will be effective for annual reporting periods beginning after 15 December 2017, including interim reporting periods within that reporting period, which for the Company is the first quarter of fiscal 2018, using one of two prescribed retrospective methods. The Company is currently in the process of evaluating the impact of adoption on its consolidated financial statements and related disclosures. The Company has not yet selected a transition method, nor has it determined the effect of the standard on its ongoing financial reporting. In May 2014, the FASB issued ASU No. 2014-9, Revenue from Contracts with Customers, amending revenue recognition guidance and requiring more detailed disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In August 2014, the FASB issued ASU No. 2014-15, to communicate amendments to FASB Accounting Standards Codification Subtopic 205-40, "Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern." The ASU requires management to evaluate relevant conditions, events and certain management plans that are known or reasonably knowable as of the evaluation date when determining whether substantial doubt about an entity's ability to continue as a going concern exists. Management will be required to make this evaluation for both annual and interim reporting periods. Management will need to make certain disclosures if it concludes that substantial doubt exists or when it plans to alleviate substantial doubt about the entity's ability to continue as a going concern. The standard is effective for annual periods ending after December 15, 2016 and for interim reporting periods starting in the first quarter of 2017. Early adoption is permitted. The Company does not expect the adoption of the guidance to have a material impact on its consolidated financial statements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2015

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of inventories

Inventories consisted of the following as of December 31, 2014 and 2015:



	2014		2015
Raw materials and spare parts	$	31,389	$	25,113
Work in process	3,292		973
Finished goods	15		3,203
Total	$	34,696	$	29,289

Schedule of the Company's intangible assets

The Company's intangible assets as of December 31, 2014 and 2015 were as follows:



	2014			2015
Technology	$	54,400		$	54,400
Customer relationships	16,430			16,576
Acquired contracts	3,694			3,694
Trademark and trade names	8,200			8,200
Non-compete agreements	2,060			2,060
Total intangible assets	84,784			84,930
Less accumulated amortization	(24,384		)	(30,442		)
Foreign currency rate change	(5,039		)	(11,844		)
Net intangible assets	$	55,361		$	42,644

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)	2013		2014		2015
Volume related revenue	$	195,328	$	247,899	$	260,629
Clean Energy Compression	77,485		84,775		54,497
Station Construction Project Sales	27,098		67,392		37,830
VETC	45,439		28,359		30,986
Other	7,125		515		378
	$	352,475	$	428,940	$	384,320

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:



	2013	2014	2015
Stock options	11,526,998	11,486,301	11,487,938
Warrants	2,130,682	6,130,682	2,130,682
Convertibles notes	35,185,979	35,185,979	35,185,979
Restricted stock units	1,590,836	2,591,752	3,419,776

Acquisitions and Divestitures (Tables)

12 Months Ended

Dec. 31, 2015

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

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



Current assets	$	4,475
Property, plant and equipment	1,369
Identifiable intangible assets	600
Goodwill	16,555
Total assets acquired	22,999
Current liabilities assumed	(1,984		)
Total purchase price	$	21,015

Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2015

Cash and Cash Equivalents [Abstract]

Schedule of components of restricted cash

Restricted cash consisted of the following as of December 31, 2014 and 2015:



	December 31, 2014		December 31, 2015
Short-term restricted cash:
Standby letters of credit	$	1,753	$	1,631
Canton Bonds (see note 9)	4,259		2,609
Total short-term restricted cash	$	6,012	$	4,240

Investments (Tables)

12 Months Ended

Dec. 31, 2015

Investments, Debt and Equity Securities [Abstract]

Summary of Short-term investments

Short-term investments as of December 31, 2014 are summarized as follows:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds & notes	$	38,668	$	(16	)	$	38,652
Zero coupon bonds	3,308		(2		)	3,306
Corporate bonds	45,274		(41		)	45,233
Certificate of deposits	35,355		—			35,355
Total short-term investments	$	122,605	$	(59	)	$	122,546

Short-term investments as of December 31, 2015 are summarized as follows:



	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

Other Receivables (Tables)

12 Months Ended

Dec. 31, 2015

Receivables [Abstract]

Schedule of other receivables

Other receivables at December 31, 2014 and 2015 consisted of the following:



	2014		2015
Loans to customers to finance vehicle purchases	$	8,257	$	10,531
Accrued customer billings	10,143		7,106
Fuel tax and carbon credits	34,250		40,730
Other	3,573		2,300
	$	56,223	$	60,667

Land, Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2015

Property, Plant and Equipment [Abstract]

Summary of land, property and equipment

Land, property and equipment at December 31, 2014 and 2015 are summarized as follows:



	2014			2015
Land	$	2,858		$	2,858
LNG liquefaction plants	94,636			94,634
RNG plants	45,359			46,397
Station equipment	265,086			316,258
LNG trailers	40,067			50,414
Other equipment	74,796			83,687
Construction in progress	163,737			139,586
	686,539			733,834
Less accumulated depreciation	(172,270		)	(217,510		)
	$	514,269		$	516,324

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2015

Payables and Accruals [Abstract]

Schedule of accrued liabilities

Accrued liabilities at December 31, 2014 and 2015 consisted of the following:



	2014		2015
Salaries and wages	$	9,041	$	9,537
Accrued gas and equipment purchases	12,340		14,133
Accrued property and other taxes	5,178		5,344
Accrued professional fees	1,084		1,105
Accrued employee benefits	3,208		3,042
Accrued warranty liability	2,302		1,826
Accrued interest	3,748		3,718
Other	19,859		20,377
	$	56,760	$	59,082

Debt (Tables)

12 Months Ended

Dec. 31, 2015

Debt Disclosure [Abstract]

Schedule of long-term debt

Debt and capital lease obligations at December 31, 2014 and 2015 consisted of the following and are further discussed below:



	December 31, 2014			December 31, 2015
7.5% Notes(1)	$	150,000		$	150,000
SLG Notes	145,000			145,000
5.25% Notes	250,000			250,000
Canton Bonds	12,150			10,910
Capital lease obligations	2,692			6,448
Other debt	10,828			10,056
Total debt and capital lease obligations	570,670			572,414
Less amounts due within one year	(4,846		)	(150,129		)
Total long-term debt and capital lease obligations	$	565,824		$	422,285

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

The following is a summary of aggregate maturities of long-term debt and capital lease obligations for each of the years ending December 31:



	2016		2017		2018		2019		2020		Thereafter
7.5% Notes(1)	—		—		50,000		50,000		50,000		—
SLG Notes	145,000		—		—		—		—		—
5.25% Notes	—		—		250,000		—		—		—
Canton Bonds	1,390		1,420		1,460		1,555		1,665		3,420
Capital lease obligations	1,294		1,329		1,427		1,425		440		533
Other debt	2,445		2,706		2,227		1,671		1,007		—
Total	$	150,129	$	5,455	$	305,114	$	54,651	$	53,112	$	3,953

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2015

Stockholders' Equity Note [Abstract]

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



	Years Ended December 31,
	2013		2014		2015
Stock-based compensation expense, net of $0 tax in 2013, 2014 and 2015	$	23,008	$	11,514	$	10,779

Summary of stock option activity

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 at December 31, 2012	12,083,677		$	11.75
Granted	98,500		13.47
Exercised	(119,349	)	5.24
Forfeited or Expired	(535,830	)	12.64
Options Outstanding at 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 at 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 at December 31, 2015	11,487,938		$	11.44	4.93	—
Options Exercisable at December 31, 2015	9,736,608		$	12.32	4.13	—

The following table summarizes Clean Energy Renewables' unit option activity during the year ended December 31, 2015:



	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Options Outstanding at December 31, 2012	—		$	—
Options granted	115,000		40.80
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding at December 31, 2013	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited or expired	—		—
Options Outstanding at December 31, 2014	115,000		$	40.80
Options granted	—		—		—	—
Options exercised	—		—
Options forfeited or expired	(7,000	)	40.80
Options Outstanding and non-vested, December 31, 2015	108,000		$	40.80		$	—

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,
	2013	2014	2015
Dividend yield	0.0%	0.0%	0.0%
Expected volatility	51.0% to 55.6%	52.3% to 67.0%	59.2% to 72.0%
Risk-free interest rate	1.0% to 1.9%	1.1% to 1.8%	1.7% to 1.8%
Expected life in years	6.0	6.0	6.0

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



Dividend yield	0.0	%
Expected volatility	96.4	%
Risk-free interest rate	1.9	%
Expected life in years	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 at December 31, 2012	—		$	—
Granted	45,836		13.09
Vested	—		—
Forfeited or expired	—		—
RSU Outstanding at December 31, 2013	45,836		$	13.09
Granted	792,500		5.54
Vested	(15,584	)	13.09
Forfeited or expired	—		—
RSU Outstanding at 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 and Unvested at December 31, 2015	1,650,776		$	5.50	2.19

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 at December 31, 2012	1,545,000		$	11.42
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding at December 31, 2013	1,545,000		$	11.42
Granted	489,500		8.26
Vested	—		—
Forfeited or Expired	(265,500	)	10.62
RSU Outstanding at December 31, 2014	1,769,000		$	10.67
Granted	—		—
Vested	—		—
Forfeited or Expired	—		—
RSU Outstanding and Unvested at December 31, 2015	1,769,000		$	10.67	0.58

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income Tax Disclosure [Abstract]

Schedule of components of loss before income taxes

The components of loss before income taxes for the years ended December 31, 2013, 2014 and 2015 are as follows:



	2013			2014			2015
U.S.	$	(40,195	)	$	(64,913	)	$	(111,437	)
Foreign	(23,009		)	(24,871		)	(22,407		)
	$	(63,204	)	$	(89,784	)	$	(133,844	)

Schedule of provision for income taxes

The provision for income taxes consists of the following:



	2013		2014			2015
Current:
Federal	$	87	$	190		$	9
State	310		238			248
Foreign	2,910		1,017			912
Total current	3,307		1,445			1,169
Deferred:
Federal	134		29			337
State	47		(10		)	71
Foreign	227		(389		)	37
Total deferred	408		(370		)	445
Total	$	3,715	$	1,075		$	1,614

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

Income tax expense for the years ended December 31, 2013, 2014 and 2015 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following:



	2013			2014			2015
Computed expected tax (benefit)	$	(22,121	)	$	(30,415	)	$	(46,846	)
Nondeductible expenses	7,216			10,690			24,998
Tax rate differential on foreign earnings	2,993			5,733			3,701
Basis difference on sale	(6,457		)	—			—
Tax credits	(35,604		)	(8,286		)	(9,988		)
Other	82			(1,121		)	(372		)
Change in valuation allowance	57,606			24,474			30,121
Total tax expense	$	3,715		$	1,075		$	1,614

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, 2014 and 2015 are as follows:



	2014			2015
Deferred tax assets:
Accrued expenses	$	4,834		$	4,512
Sales-type leases	908			154
Alternative minimum tax and general business credits	4,966			5,780
Derivative loss	527			—
Stock option expense	21,741			23,113
Other	2,596			2,283
Loss carryforwards	147,706			174,157
Total deferred tax assets	183,278			209,999
Less valuation allowance	(160,436		)	(189,203		)
Net deferred tax assets	22,842			20,796
Deferred tax liabilities:
Depreciation and amortization	(20,028		)	(17,398		)
Goodwill	(3,864		)	(4,600		)
Partnership income	—			(293		)
Total deferred tax liabilities	(23,892		)	(22,291		)
Net deferred tax liabilities	$	(1,050	)	$	(1,495	)

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, 2013, 2014 and 2015:



Unrecognized tax benefit—December 31, 2013	$	17,398
Gross increases—tax positions in current year	4,722
Gross increases—tax positions in prior years	(146		)
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

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2015

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, 2015:



Fiscal year:
2016	$	7,998
2017	7,750
2018	6,106
2019	5,909
2020	5,388
Thereafter	20,874
Total future minimum lease payments	$	54,025

Geographic Information (Tables)

12 Months Ended

Dec. 31, 2015

Segment Reporting [Abstract]

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



	2013			2014			2015
Revenue:
United States	$	292,250		$	360,881		$	330,003
Canada	13,922			16,241			21,818
Other	46,303			51,818			32,499
Total revenue	$	352,475		$	428,940		$	384,320
Operating income (loss):
United States	$	(44,797	)	$	(41,543	)	$	(33,067	)
Canada	(1,594		)	(3,087		)	(4,980		)
Other	(5,300		)	(9,734		)	(3,576		)
Total operating income (loss)	$	(51,691	)	$	(54,364	)	$	(41,623	)
Long-lived assets:
United States	$	544,638		$	582,028		$	582,644
Canada	103,997			85,984			68,292
Other	7,537			6,854			5,693
Total long-lived assets	$	656,172		$	674,866		$	656,629

Capitalized Lease Obligation and Receivables (Tables)

12 Months Ended

Dec. 31, 2015

Leases [Abstract]

Schedule of future payments under the capital leases

At December 31, 2015, future payments under these capital leases are as follows:



2016	$	1,688
2017	1,738
2018	1,739
2019	1,633
2020	660
Thereafter	633
Total minimum lease payments	8,091
Less amount representing interest	(1,643		)
Future minimum lease payments	6,448
Less current portion	(1,294		)
Capital lease obligations, less current portion	$	5,154

Schedule of future receipts under the leases

At December 31, 2015, future receipts under these leases are as follows:



2016	$	563
2017	380
2018	251
2019	186
2020	186
Thereafter	1,798
Total	3,364
Less amount representing interest	(1,575		)
	$	1,789

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2015

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, 2014 and December 31, 2015, respectively:



Description	Balance at December 31, 2014		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Certificate of deposits	$	35,355	$	—	$	35,355	$	—
Municipal bonds and notes	38,652		—		38,652		—
Zero coupon bonds	3,306		—		3,306		—
Corporate bonds	45,233		—		45,233		—
Liabilities:
Warrants(2)	1,416		—		—		1,416



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

_______________________________________________________________________________


(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	2014			2015
Beginning Balance	$	7,164		$	2,046
Gain included in earnings	(5,748		)	(1,414		)
Warrants acquired from NG Advantage acquisition (see note 11)	630			—
Ending Balance	$	2,046		$	632

Schedule of fair value measurements of liabilities which fall within level 3 of the fair value hierarchy

The Company estimated the fair value of its Series I Warrant liability using the Black-Scholes option pricing model based on the following inputs as of December 31, 2015:



Unobservable Inputs	Range or Weighted Average
Current market price of the Company's common stock	$	3.60
Exercise price of the warrant	$	12.54
Dividend yield	0.00		%
Remaining term of the warrant	0.33
Implied volatility of the Company's common stock	83.05		%
Assumed discount rate	0.33		%

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Inventories
Raw materials and spare parts	$ 25,113	$ 31,389
Work in process	973	3,292
Finished goods	3,203	15
Total	29,289	34,696
Property and Equipment
Proceeds from grants	$ 4,292	$ 959	$ 3,114
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
LNG 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	Jun. 28, 2013	May 06, 2013	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill and intangible assets
Impairment charges for goodwill or indefinite lived intangible asset							$ 0	$ 0
Long-lived intangible impairment						$ 0	4,772,000	0
Foreign currency translation gains (losses) included in goodwill balances						(6,578,000)	(3,687,000)
Total intangible assets					$ 84,784,000	84,930,000	84,784,000
Less accumulated amortization					(24,384,000)	(30,442,000)	(24,384,000)
Foreign currency rate change						(11,844,000)	(5,039,000)
Net intangible assets					55,361,000	42,644,000	55,361,000
Carrying value of trademark of indefinite useful life					1,200,000		1,200,000
Amortization expense						5,539,000	7,024,000	$ 9,016,000
Estimated amortization expense
2016						4,788,000
2017						4,574,000
2018						4,011,000
2019						2,953,000
2020						2,933,000
Thereafter						23,385,000
Technology
Goodwill and intangible assets
Total intangible assets					54,400,000	$ 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					16,430,000	$ 16,576,000	16,430,000
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					3,694,000	$ 3,694,000	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	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						20 years
Non-compete agreements
Goodwill and intangible assets
Estimated useful lives						3 years
Total intangible assets					2,060,000	$ 2,060,000	$ 2,060,000
MGES
Goodwill and intangible assets
Identifiable intangible assets, customer relationships and trade names				$ 600,000
Increase in goodwill on acquisition				$ 16,555,000
MGES \| Minimum
Goodwill and intangible assets
Estimated useful lives				1 year
MGES \| Maximum
Goodwill and intangible assets
Estimated useful lives				6 years
NG Advantage
Goodwill and intangible assets
Identifiable intangible assets, customer relationships and trade names		$ 5,600,000
Increase in goodwill on acquisition		$ 21,070,000
NG Advantage \| Minimum
Goodwill and intangible assets
Estimated useful lives		4 years
NG Advantage \| Maximum
Goodwill and intangible assets
Estimated useful lives		7 years
NG Advantage \| Customer relationships and tradenames
Goodwill and intangible assets
Identifiable intangible assets, customer relationships and trade names		$ 5,600,000
IMW
Goodwill and intangible assets
Long-lived intangible impairment					$ 4,772,000
BAF
Goodwill and intangible assets
Reduction in goodwill on sale			$ 774,000
Dallas Clean Energy, LLC
Goodwill and intangible assets
Reduction in goodwill on sale	$ 7,205,000

Summary of Significant Accounting Policies - Warranty Liability (Details) $ in Thousands	12 Months Ended				111 Months Ended
	Dec. 31, 2016 $ / gal	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 $ / gal
Alternative Fuels Excise Tax Credit
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent) \| $ / gal					0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon) \| $ / gal					0.50
VETC credits recognized as revenue \| $		$ 30,986	$ 28,359	$ 45,439
VETC credits related to prior periods \| $				20,800
Advertising Costs
Advertising expense \| $		44	439	1,391
Foreign Currency Translation
Foreign exchange transactions gains (losses) \| $		$ 975	$ (3,188)	$ (1,196)
Forecast
Alternative Fuels Excise Tax Credit
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent) \| $ / gal	0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon) \| $ / gal	0.50

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 384,320	$ 428,940	$ 352,475
Volume related revenue
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	260,600	247,900	195,300
Clean Energy Compression
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	54,497	84,800	77,500
Station Construction Project Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	37,830	67,400	27,100
VETC
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	30,986	28,400	45,400
Other
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 378	$ 500	$ 7,100

Summary of Significant Accounting Policies - Net Loss Per Share (Details) - shares				4 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 11, 2014	Nov. 07, 2012	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Loss Per Share
Number of shares that will not vest and will not become exercisable					4,000,000
GE Credit Agreement \| GE Warrant
Net Loss Per Share
Number of shares exercisable in the period	500,000		500,000					500,000
Common stock \| GE Credit Agreement \| GE Warrant
Net Loss Per Share
Anti-dilutive securities (in shares)		4,000,000		4,000,000
Shares issuable upon exercise included in earnings per share calculations				1,000,000
Number of shares that will not vest and will not become exercisable					4,000,000
Stock Options
Net Loss Per Share
Anti-dilutive securities (in shares)					11,487,938	11,486,301	11,526,998
Series I Warrant Liabilities
Net Loss Per Share
Common stock related to the GE Warrant included in the basic and dilutive net loss per share calculation (in shares)							5,000,000
Anti-dilutive securities (in shares)					2,130,682	6,130,682	2,130,682
Convertible Notes
Net Loss Per Share
Anti-dilutive securities (in shares)					35,185,979	35,185,979	35,185,979
Restricted Stock Units
Net Loss Per Share
Anti-dilutive securities (in shares)					3,419,776	2,591,752	1,590,836

Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Concentration of Credit Risk
Amount in excess of FDIC insurance limits	$ 40,691	$ 88,740
IMW
Concentration of Credit Risk
Maximum percentage of foreign trade receivables covered by accounts receivable insurance (up to)	90.00%

Acquisitions and Divestitures (Details)		1 Months Ended
Acquisitions and Divestitures (Details)	Jun. 28, 2013 USD ($) member shares gal	Jul. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Aug. 31, 2013 USD ($)	Jul. 31, 2013 USD ($) shares	Jun. 30, 2014 shares
BAF \| ServoTech
Divestitures
Ownership interest in ServoTech	100.00%
BAF \| Westport Parties \| Stock purchase agreement
Divestitures
Number of common stock shares to be received \| shares	816,460
Number of common stock shares received \| shares	718,485					94,914
Number of common stock Holdback Shares \| shares	97,975
Period of holdback for shares to be received	1 year
Number of Holdback Shares remaining unissued \| shares						3,000
Number of common stock shares sold \| shares					718,485
Net proceeds from sale of common stock		$ 1,727,000			$ 23,722,000
Repayment of certain intercompany indebtedness of BAF Technologies, Inc. and ServoTech Engineering Inc. (BAF) to the Company				$ 2,478,000
Value of common stock shares to be received	$ 27,221,000
Initial/adjusted gain recognized	15,498,000	13,993,000	$ 14,115,000
Write down in value of the Holdback Shares		$ 122,000	$ 1,383,000
BAF \| Westport Affiliates \| Marketing Agreement
Divestitures
Cash payable to entity under the agreement	$ 5,000,000
Term of agreement	2 years
Complimentary gasoline provided under the agreement (in gallons) \| gal	750,000
Minimum number of members to be appointed by Company to operating committee (in members) \| member	1

Acquisitions and Divestitures - MGES Acquisition (Details) - USD ($)		1 Months Ended
	May 06, 2013	Aug. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Allocation of aggregate purchase price
Goodwill			$ 91,967,000	$ 98,726,000
MGES
Allocation of aggregate purchase price
Current assets	$ 4,475,000
Property, plant and equipment	1,369,000
Identifiable intangible assets	600,000
Goodwill	16,555,000
Total assets acquired	22,999,000
Current liabilities assumed	(1,984,000)
Total purchase price	$ 21,015,000
MGES \| Minimum
Allocation of aggregate purchase price
Identifiable intangible assets, estimated useful lives	1 year
MGES \| Maximum
Allocation of aggregate purchase price
Identifiable intangible assets, estimated useful lives	6 years
MGES \| Stock purchase agreement
Acquisitions
Total purchase consideration	$ 20,000,000	$ 563,000
Percentage of purchase consideration payable in cash	50.00%
Percentage of purchase consideration paid in shares	50.00%
Cash payment	$ 9,000,000
Number of common stock shares issued	761,545
Amount retained for final purchase price adjustment	$ 1,000,000

Acquisitions and Divestitures - NG Advantage (Details) $ in Thousands	Oct. 14, 2014 USD ($) payment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Allocation of aggregate purchase price
Goodwill		$ 91,967	$ 98,726
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
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 (as a percent)	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	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2015
Divestitures
Gain from sale of subsidiary						$ 937	$ 11,998	$ 14,115
Goodwill			$ 98,726		$ 98,726	91,967	$ 98,726
Dallas Clean Energy, LLC \| Cambrian
Divestitures
Cash consideration received on sale of subsidiary			$ 40,588	$ 6,992
Dallas Clean Energy, LLC \| Cambrian \| Mavrix
Divestitures
Percentage of ownership sold	51.00%	19.00%
Ownership interest		70.00%
Additional consideration that may be received prior to August 1, 2015									$ 1,118
Gain from sale of subsidiary		$ 12,935			$ 11,998	$ 937
Goodwill	$ 7,386

Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Restricted Cash
Short-term restricted cash	$ 4,240	$ 6,012
Canton Bonds
Restricted Cash
Short-term restricted cash	2,609	4,259
Standby letters of credit
Restricted Cash
Short-term restricted cash	$ 1,631	$ 1,753

Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Short-term investments
Amortized Cost	$ 103,029	$ 122,605
Gross Unrealized Losses	(85)	(59)
Estimated Fair Value	102,944	122,546
Municipal bonds and notes
Short-term investments
Amortized Cost	16,797	38,668
Gross Unrealized Losses	(7)	(16)
Estimated Fair Value	16,790	38,652
Zero coupon bonds
Short-term investments
Amortized Cost	500	3,308
Gross Unrealized Losses	(1)	(2)
Estimated Fair Value	499	3,306
Corporate bonds
Short-term investments
Amortized Cost	37,181	45,274
Gross Unrealized Losses	(77)	(41)
Estimated Fair Value	37,104	45,233
Certificates of deposit
Short-term investments
Amortized Cost	48,551	35,355
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 48,551	$ 35,355

Other Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Other Receivables
Other receivables	$ 60,667	$ 56,223
Loans to customers to finance vehicle purchases
Other Receivables
Other receivables	10,531	8,257
Accrued customer billings
Other Receivables
Other receivables	7,106	10,143
Fuel tax and carbon credits
Other Receivables
Other receivables	40,730	34,250
Other
Other Receivables
Other receivables	$ 2,300	$ 3,573

Land, Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Land, Property and Equipment
Land, property and equipment, gross	$ 733,834	$ 686,539
Less: accumulated depreciation	(217,510)	(172,270)
Land, property and equipment, net	516,324	514,269
Capitalized software costs, net	22,886	21,004
Accumulated amortization on the capitalized software costs	13,793	10,740
Amortization expense related to the capitalized software costs	3,053	2,993	$ 3,079
Amount included in accounts payable balances	5,955	11,032
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	94,636
RNG plants
Land, Property and Equipment
Land, property and equipment, gross	46,397	45,359
Station equipment
Land, Property and Equipment
Land, property and equipment, gross	316,258	265,086
LNG trailers
Land, Property and Equipment
Land, property and equipment, gross	50,414	40,067
Other equipment
Land, Property and Equipment
Land, property and equipment, gross	83,687	74,796
Construction in progress
Land, Property and Equipment
Land, property and equipment, gross	$ 139,586	$ 163,737

Investment in Other Entities (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 16, 2014
Equity method investments
Dividend distribution		$ 0	$ 0	$ 1,091
Gain on sale recognized		0	0	4,705
Loss from investment		(815)	(490)	$ (76)
Peru JV
Equity method investments
Ownership interest sold	$ 6,119
Dividend distribution	1,091
Gain on sale recognized	$ 4,705
Mansfield Energy Corp
Equity method investments
Ownership interest					50.00%
Loss from investment		490	815
Investment balance		$ 5,510	$ 4,695

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Salaries and wages	$ 9,537	$ 9,041
Accrued gas and equipment purchases	14,133	12,340
Accrued property and other taxes	5,344	5,178
Accrued professional fees	1,105	1,084
Accrued employee benefits	3,042	3,208
Accrued warranty liability	1,826	2,302
Accrued interest	3,718	3,748
Other	20,377	19,859
Total	$ 59,082	$ 56,760

Debt - Debt and Capital Lease Obligations (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Long-term debt
Total debt and capital lease obligations	$ 572,414,000	$ 570,670,000
Less amounts due within one year	(150,129,000)	(4,846,000)
Total long-term debt and capital lease obligations	422,285,000	565,824,000
Summary of aggregate maturities of long-term debt
2016	150,129,000
2017	5,455,000
2018	305,114,000
2019	54,651,000
2020	53,112,000
Thereafter	3,953,000
7.5% Notes
Long-term debt
Total debt and capital lease obligations	150,000,000	150,000,000
Summary of aggregate maturities of long-term debt
2016	0
2017	0
2018	50,000,000
2019	50,000,000
2020	50,000,000
Thereafter	0
7.5% Notes \| Long-Term Debt, Related Party \| T. Boone Pickens
Long-term debt
Total debt and capital lease obligations		65,000,000
SLG Notes
Long-term debt
Total debt and capital lease obligations	145,000,000	145,000,000
Summary of aggregate maturities of long-term debt
2016	145,000,000
2017	0
2018	0
2019	0
2020	0
Thereafter	0
5.25% Notes
Long-term debt
Total debt and capital lease obligations	250,000,000	250,000,000
Summary of aggregate maturities of long-term debt
2016	0
2017	0
2018	250,000,000
2019	0
2020	0
Thereafter	0
Canton Bonds
Long-term debt
Total debt and capital lease obligations	10,910,000	12,150,000
Summary of aggregate maturities of long-term debt
2016	1,390,000
2017	1,420,000
2018	1,460,000
2019	1,555,000
2020	1,665,000
Thereafter	3,420,000
Capital lease obligations
Long-term debt
Total debt and capital lease obligations	6,448,000	2,692,000
Summary of aggregate maturities of long-term debt
2016	1,294,000
2017	1,329,000
2018	1,427,000
2019	1,425,000
2020	440,000
Thereafter	533,000
Other debt
Long-term debt
Total debt and capital lease obligations	10,056,000	$ 10,828,000
Summary of aggregate maturities of long-term debt
2016	2,445,000
2017	2,706,000
2018	2,227,000
2019	1,671,000
2020	1,007,000
Thereafter	$ 0

Debt - 7.5 Notes (Details) - 7.5% Notes	Aug. 27, 2013 USD ($) shares	Jun. 14, 2013 USD ($) day $ / shares	Jul. 11, 2011 USD ($) note	Dec. 31, 2015 USD ($)
Long-term debt
Financing commitment received			$ 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
Interest rate			7.50%
T. Boone Pickens
Long-term debt
Aggregate principal amount	$ 65,000,000
T. Boone Pickens \| Common stock
Long-term debt
Number of shares of common stock into which Notes are convertible \| shares	4,113,924
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
Long term restricted cash related to debt instrument				$ 0
Green Energy Investment Holdings, LLC
Long-term debt
Principal amount transferred	$ 5,000,000
Aggregate principal amount	$ 80,000,000
Green Energy Investment Holdings, LLC \| Common stock
Long-term debt
Number of shares of common stock into which Notes are convertible \| shares	5,063,291
Other Third Parties
Long-term debt
Aggregate principal amount	$ 5,000,000
Other Third Parties \| Common stock
Long-term debt
Number of shares of common stock into which Notes are convertible \| shares	316,456

Debt - SLG Notes (Details) - SLG Notes			1 Months Ended	2 Months Ended
Debt - SLG Notes (Details) - SLG Notes	Feb. 29, 2016 USD ($)	Aug. 24, 2011 USD ($) day $ / shares	Apr. 30, 2012 USD ($) shares	Feb. 28, 2013 USD ($) shares	Mar. 01, 2012 USD ($)
Long-term debt
Financing commitment received		$ 150,000,000
Interest rate		7.50%
Conversion price of shares (in dollars per share) \| $ / shares		$ 15.00
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		5 years
Subsequent Event
Long-term debt
Repayments of debt	$ 60,000,000
Aggregate principal amount	$ 145,000,000
Common stock
Long-term debt
Amount of principal and accrued interest under debt conversion			$ 1,003,000	$ 4,030,000
Common stock issued upon conversion of debt (in shares) \| shares			66,888	268,664
Baytree Investments (Mauritius) Pte Ltd
Long-term debt
Principal amount transferred					$ 24,000,000

Debt - 5.25% (Details)		1 Months Ended	12 Months Ended
Debt - 5.25% (Details)	Feb. 18, 2016 USD ($)	Sep. 30, 2013 USD ($) day $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Long-term debt
Payment of certain debt issuance costs			$ 0	$ 896,000	$ 9,130,000
5.25% Notes
Long-term debt
Payment of certain debt issuance costs		$ 7,805,000
Amount of advance funded		$ 242,195,000
Interest rate		5.25%
Denominator in calculation of shares per principal amount in debt conversion		$ 1,000
Common stock issued upon conversion of debt (in shares) \| shares		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%
5.25% Notes \| Subsequent Event
Long-term debt
Repayments of debt	$ 16,761,000
Amount of par value debt retired	32,500,000
Aggregate principal amount	$ 250,000,000
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 - Canton Bonds (Details) - USD ($)	12 Months Ended
Debt - Canton Bonds (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Mar. 19, 2014
Long-term debt
Maximum effective interest rate (up to)	19.33%
Weighted average interest rate	6.35%	6.80%
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 - GE Loans and Other Agreements (Details)					3 Months Ended	12 Months Ended			24 Months Ended
Debt - GE Loans and Other Agreements (Details)	Feb. 29, 2016 USD ($)	Dec. 31, 2014 shares	Apr. 25, 2013 USD ($)	Nov. 07, 2012 USD ($) subsidiary plant $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2013 USD ($)	Dec. 31, 2012 shares	Dec. 31, 2014 USD ($)
Long-term debt
Number of shares that will not vest and will not become exercisable \| shares					4,000,000	4,000,000
Common stock
Long-term debt
Exercise price of the warrant (in dollars per share) \| $ / shares				$ 0.01
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				$ 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		$ 1,014,000
Write off of deferred financing costs					$ 54,900,000
GE Credit Agreement \| GE Warrant
Long-term debt
Number of shares exercisable in the period \| shares		500,000		500,000				500,000
GE Credit Agreement \| GE Warrant \| Common stock
Long-term debt
Number of shares that can be purchased upon exercise of warrants \| 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 \| 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
Plains Capital Bank Credit Agreement \| Subsequent Event
Long-term debt
Line of credit limit	$ 50,000,000
Interest rate	2.70%
Plains Capital Bank Credit Agreement \| Subsequent Event \| LIBOR
Long-term debt
Basis spread on variable rate	2.30%

Derivative Transactions (Details) - USD ($)	12 Months Ended
Derivative Transactions (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Transactions
Unrealized gains on derivative instruments recognized in other comprehensive income loss	$ 0	$ 0	$ 108,000
Derivative gain	(1,414,000)	(5,748,000)	(938,000)
Forward exchange contracts
Derivative Transactions
Derivative liability	$ 0
Natural gas futures contracts
Derivative Transactions
Derivative liability			107,000
Unrealized gains on derivative instruments recognized in other comprehensive income loss		108,000	2,151,000
Derivative gain		$ 65,000	2,370,000
Natural gas futures contracts \| Prepaid expenses and other current assets
Derivative Transactions
Derivative asset			5,000
Natural gas futures contracts \| Accrued liabilities
Derivative Transactions
Derivative liability			$ 112,000

Stockholders' Equity (Details)	Dec. 31, 2015 class vote $ / shares shares	Dec. 31, 2014 $ / 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	225,000,000
Common stock, Authorized shares	224,000,000	224,000,000
Preferred stock, Authorized 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)							1 Months Ended	2 Months Ended	3 Months Ended	12 Months Ended
	Nov. 11, 2015 USD ($) shares	Dec. 31, 2014 USD ($) $ / shares shares	Nov. 07, 2012 USD ($) $ / shares shares	Nov. 10, 2010 USD ($) Item shares	Nov. 12, 2008 $ / shares shares	Nov. 03, 2008 USD ($) $ / shares shares	Apr. 30, 2012 shares	Feb. 28, 2013 shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2012 shares	Oct. 28, 2008 warrant unit $ / shares shares
Stockholders' equity
Number of units authorized under the Placement Agent Agreement \| unit													4,419,192
Number of shares of the company's common stock in a unit													1
Number of warrants in a unit \| warrant													1
Unit price (in dollars per unit) \| $ / shares													$ 7.92
Proceeds after deducting the placement agent's fees and other offering expenses related to the sale of units \| $						$ 32,484,000
Number of shares that will not vest and will not become exercisable									4,000,000	4,000,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.0001							$ 0.0001	$ 0.0001	$ 0.0001
Common stock, issued shares		90,203,344							92,382,717	92,382,717	90,203,344
Equity distribution agreement \| Citigroup
Stockholders' equity
Common stock, amount authorized (up to) \| $	$ 75,000,000
Proceeds from issuance of common stock \| $	6,500,000
Stock issuance costs \| $	$ 500,000
Common stock, issued shares	1,561,902
GE Credit Agreement
Stockholders' equity
Write off of deferred financing costs \| $									$ 54,900,000
Common stock
Stockholders' equity
Number of shares issued for units under Placement Agent Agreement						4,419,192
Proceeds after deducting the placement agent's fees and other offering expenses related to the sale of units \| $						$ 19,166,000
Exercise price of the warrant (in dollars per share) \| $ / shares			$ 0.01
Common stock \| SLG Notes
Stockholders' equity
Common stock issued upon conversion of debt (in shares)							66,888	268,664
Series I Warrants
Stockholders' equity
Proceeds after deducting the placement agent's fees and other offering expenses related to the sale of units \| $						$ 9,745,000
Period from which warrants are exercisable						6 months
Warrant expiration term						7 years
Exercise price of the warrant (in dollars per share) \| $ / shares						$ 12.54
Series II Warrant
Stockholders' equity
Proceeds after deducting the placement agent's fees and other offering expenses related to the sale of units \| $						$ 3,573,000
Exercise price of the warrant (in dollars per share) \| $ / shares					$ 0.01
Total number of warrants exercised (in shares)					1,134,759
Amended Series I Warrant
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants				1,183,712
Number of holders of warrants entering into amendment with the company \| Item				1
Amount paid to holders on modification of term \| $				$ 3,172,000
Total number of warrants exercised (in shares)				1,183,712
Total value of warrants exercised \| $				$ 15,009,000
Gain on exercise of warrants \| $				$ 3,208,000
GE Warrant \| GE Credit Agreement
Stockholders' equity
Number of shares exercisable in the period		500,000	500,000									500,000
Warrants outstanding, not exercisable (in shares)									4,000,000	4,000,000
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			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									4,000,000	4,000,000
NG Advantage Warrants \| NG Advantage
Stockholders' equity
Fair value of warrants outstanding \| $		$ 630,000							$ 561,000	$ 561,000	$ 630,000
Gain fromt he change in fair value \| $										$ 69,000	$ 0
NG Advantage Warrants \| Common stock \| NG Advantage
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants									127,200	127,200
Maximum \| Series I Warrants
Stockholders' equity
Number of shares that can be purchased upon exercise of each warrant													0.75
Number of shares that can be purchased upon exercise of warrants						3,314,394
Maximum \| Series II Warrant
Stockholders' equity
Number of shares that can be purchased upon exercise of each warrant													0.2571
Number of shares that can be purchased upon exercise of warrants						1,136,364
Maximum \| GE Warrant \| GE Credit Agreement
Stockholders' equity
Percentage of market price below which stock sale is a "Dilutive Issuance"			80.00%

Stockholders' Equity - Stock-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock-based compensation
Stock-based compensation expense, net of $0 tax in 2013, 2014 and 2015	$ 10,779,000	$ 11,514,000	$ 23,008,000
Stock-based compensation expense, tax	$ 0	0	0
2002 Plan \| Stock Options
Stock-based compensation
Award term	10 years
Percentage ownership of stock for which the option purchase price will be 110% of fair market	10.00%
Vesting period	3 years
2002 Plan \| Stock Options \| Minimum
Stock-based compensation
Purchase price of options expressed as percentage of fair market value of common stock	100.00%
Purchase price of options granted to 10% stockholders expressed as percentage of fair market value of common stock (not less than)	110.00%
2006 Plan \| Stock Options
Stock-based compensation
Stock-based compensation expense	$ 5,195,000	$ 7,286,000	$ 13,751,000
Number of shares reserved for issuance	19,890,500
Number of shares available for future grant	262,461
Number of Shares
Outstanding at the beginning of the period	11,486,301	11,526,998	12,083,677
Options granted	1,415,200	957,000	98,500
Options exercised	(608,279)	(468,279)	(119,349)
Options forfeited or expired	(805,284)	(529,418)	(535,830)
Outstanding at the end of the period	11,487,938	11,486,301	11,526,998
Exercisable at the end of the period	9,736,608
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 11.91	$ 11.79	$ 11.75
Options granted (in dollars per share)	5.39	10.23	13.47
Options exercised (in dollars per share)	2.96	3.78	5.24
Options forfeited or expired (in dollars per share)	14.04	13.40	12.64
Outstanding at the end of the period (in dollars per share)	11.44	$ 11.91	$ 11.79
Exercisable at the end of the period (in dollars per share)	$ 12.32
Additional option disclosures
Options outstanding at the end of the period, weighted average remaining contractual term	4 years 11 months 5 days
Options exercisable at the end of the period, weighted average remaining contractual term	4 years 1 month 17 days
Options outstanding at the end of the period, aggregate intrinsic value	$ 0
Options exercisable at the end of the period, aggregate intrinsic value	0
Other disclosures
Total unrecognized compensation cost related to non-vested shares	$ 4,907,000
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized	2 years 3 months 11 days
Total fair value of shares vested	$ 7,958,000
Intrinsic value of all options exercised	$ 2,197,000	$ 2,568,000	$ 935,000
Weighted-average assumption used for grants
Dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	59.20%	52.30%	51.00%
Expected volatility, maximum	72.00%	67.00%	55.60%
Risk-free interest rate, minimum	1.70%	1.10%	1.00%
Risk-free interest rate, maximum	1.80%	1.80%	1.90%
Expected life	6 years	6 years	6 years
Stock-based compensation
Weighted-average grant date fair values of options granted	$ 3.29	$ 5.32	$ 6.86

Stockholders' Equity - Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 25, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Market-Based RSUs
Stock-based compensation
Number of shares of common stock to be received for each performance unit		1
Period from the date of grant after which awards will be automatically forfeited if the Stock Price Condition is not satisfied		4 years
Number of Shares
Outstanding at the beginning of the period		1,769,000	1,545,000	1,545,000
RSUs granted		0	489,500	0
RSUs vested		0	0	0
RSUs forfeited or expired		0	(265,500)	0
Outstanding at the end of the period		1,769,000	1,769,000	1,545,000
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)		$ 10.67	$ 11.42	$ 11.42
RSUs granted (in dollars per share)		0.00	8.26	0.00
RSUs vested (in dollars per share)		0	0	0
RSUs forfeited or expired (in dollars per share)		0.00	10.62	0.00
Outstanding at the end of the period (in dollars per share)		$ 10.67	$ 10.67	$ 11.42
Options outstanding at the end of the period, weighted average remaining contractual term (in dollars per share)		6 months 29 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares		$ 2,063
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized		1 month 2 days
Stock-based compensation expense		$ 1,770	$ 2,556	$ 8,821
Market-Based RSUs \| Subsequent Event
Stock-based compensation
Shares forfeited	1,215,000
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
Stock-based compensation
Vesting period		3 years
Number of Shares
Outstanding at the beginning of the period		822,752	45,836	0
RSUs granted		1,167,750	792,500	45,836
RSUs vested		(283,726)	(15,584)	0
RSUs forfeited or expired		(56,000)	0	0
Outstanding at the end of the period		1,650,776	822,752	45,836
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)		$ 5.82	$ 13.09	$ 0.00
RSUs granted (in dollars per share)		5.38	5.54	13.09
RSUs vested (in dollars per share)		5.94	13.09	0.00
RSUs forfeited or expired (in dollars per share)		5.57	0.00	0.00
Outstanding at the end of the period (in dollars per share)		$ 5.50	$ 5.82	$ 13.09
Options outstanding at the end of the period, weighted average remaining contractual term (in dollars per share)		2 years 2 months 9 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares		$ 7,535
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized		2 years 3 months
Stock-based compensation expense		$ 2,622	$ 365	$ 51
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
RSUs granted			2,034,500

Stockholders' Equity - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan $ in Thousands	12 Months Ended			32 Months Ended
	Dec. 31, 2015 USD ($) period shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 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	2,500,000			2,500,000
Stock-based compensation expense \| $	$ 50	$ 67	$ 29
Shares sold pursuant to the ESPP				152,389

Stockholders' Equity - Non-Qualified Non-Public Subsidiary Unit Options (Details) - CERF Plan - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of Shares
Outstanding at the beginning of the period		115,000	115,000	0
Options granted		0	0	115,000
Options exercised		0	0	0
Options forfeited or expired		(7,000)	0	0
Outstanding at the end of the period		108,000	115,000	115,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 40.80	$ 40.80	$ 0.00
Options granted (in dollars per share)		0.00	0.00	40.80
Options exercised (in dollars per share)		0.00	0.00	0.00
Options forfeited or expired (in dollars per share)		40.80	0.00	0.00
Outstanding at the end of the period (in dollars per share)		$ 40.80	$ 40.80	$ 40.80
Additional option disclosures
Options outstanding at the end of the period, weighted average remaining contractual term
Options outstanding at the end of the period, aggregate intrinsic value		$ 0
Stock-based compensation
Weighted-average grant date fair values of options granted	$ 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		$ 803,000
Weighted average period over which the total unrecognized compensation cost related to non-vested units is expected to be recognized		6 months
Weighted-average assumption used for grants
Dividend yield		0.00%
Expected volatility		96.40%
Risk-free interest rate		1.90%
Expected life		6 years
Stock-based compensation
Stock-based compensation expense		$ 1,115,000	$ 1,240,000	$ 356,000

Income Taxes (Details) $ in Thousands	12 Months Ended			111 Months Ended
Income Taxes (Details) $ in Thousands	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($) $ / gal
Components of loss before income taxes
U.S.	$ (111,437)	$ (64,913)	$ (40,195)
Foreign	(22,407)	(24,871)	(23,009)
Income (loss) before income taxes, total	(133,844)	(89,784)	(63,204)
Current:
Current Federal	9	190	87
Current State	248	238	310
Current Foreign	912	1,017	2,910
Total current	1,169	1,445	3,307
Deferred:
Federal	337	29	134
State	71	(10)	47
Foreign	37	(389)	227
Total deferred	445	(370)	408
Income tax expense (benefit)	$ 1,614	$ 1,075	$ 3,715
Reconciliation of the income tax provision
Federal income tax rate (as a percent)	35.00%	35.00%	35.00%
Computed expected tax expense (benefit)	$ (46,846)	$ (30,415)	$ (22,121)
Nondeductible expenses	24,998	10,690	7,216
Tax rate differential on foreign earnings	3,701	5,733	2,993
Basis difference on sale	0	0	(6,457)
Tax credits	(9,988)	(8,286)	(35,604)
Other	(372)	(1,121)	82
Change in valuation allowance	30,121	24,474	57,606
Income tax expense (benefit)	1,614	1,075	3,715
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent) \| $ / gal				0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon) \| $ / gal				0.50
Tax benefit related to exclusion of VETC associated with 2013 fuel sales	9,298	8,221	$ 7,068
Tax benefit related to exclusion of VETC associated with 2006 to 2012 fuel sales		27,497
Deferred tax assets:
Accrued expenses	4,512	4,834		$ 4,512
Sales-type leases	154	908		154
Alternative minimum tax and general business credits	5,780	4,966		5,780
Derivative loss	0	527		0
Stock option expense	23,113	21,741		23,113
Other	2,283	2,596		2,283
Loss carryforwards	174,157	147,706		174,157
Total deferred tax assets	209,999	183,278		209,999
Less valuation allowance	(189,203)	(160,436)		(189,203)
Net deferred tax assets	20,796	22,842		20,796
Deferred tax liabilities:
Depreciation and amortization	(17,398)	(20,028)		(17,398)
Goodwill	(4,600)	(3,864)		(4,600)
Partnership income	(293)	0		(293)
Total deferred tax liabilities	(22,291)	(23,892)		(22,291)
Net deferred tax liabilities	$ (1,495)	$ (1,050)		$ (1,495)

Income Taxes - Tax Credit Carryforwards (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Federal
Income Taxes
Operating loss carryforwards	$ 438,384
Federal \| General
Income Taxes
Tax credit carryforwards	5,572
State
Income Taxes
Operating loss carryforwards	336,251
Foreign
Income Taxes
Operating loss carryforwards	$ 58,090

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation allowance
Valuation allowance	$ 189,203,000	$ 160,436,000
Net increase in valuation allowance	(28,767,000)	(23,468,000)
Undistributed earnings of non-U.S. subsidiaries	2,828,000
Unrecognized tax benefits, which if recognized, would primarily affect the effective tax rate	692,000
Unrecognized tax benefits, which if recognized, would increase the net operating loss carryforwards	26,805,000
Reconciliation of the total amounts of unrecognized tax benefits
Unrecognized tax benefit at the beginning of the period	21,974,000	17,398,000
Gross increases-tax positions in current year	5,523,000	4,722,000
Gross increases-tax positions in prior years	0	(146,000)
Unrecognized tax benefit at the end of the period	27,497,000	21,974,000	$ 17,398,000
Additional information on income tax
Accrued interest expense and penalties	0	120,000
Interest expense related to uncertain tax positions	$ 58,000	$ 54,000	$ 48

Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fiscal year:
2016	$ 7,998
2017	7,750
2018	6,106
2019	5,909
2020	5,388
Thereafter	20,874
Total future minimum lease payments	54,025
Rent expense	$ 8,629	$ 10,140	$ 10,504

Commitments and Contingencies - Long-Term Take-or-Pay Natural Gas Supply Contracts (Details) gal / d in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 01, 2014 gal / d	Oct. 31, 2007 gal / d
LNG supply agreement \| DGS
Long-Term Take-or-pay Contracts
Number of gallons of LNG per day \| gal / d				65	45
Amount paid under the contract	$ 11,852	$ 14,267	$ 11,404
Fixed commitments under the contract payable in future
2017	4,579
2018	3,816
CNG supply agreement \| JTA
Fixed commitments under the contract payable in future
2016	71
2017	203
2018	313
2019	429
2020	$ 548

Geographic Information (Details) - USD ($) $ in Thousands	12 Months Ended
Geographic Information (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Geographic Information
Total revenue	$ 384,320	$ 428,940	$ 352,475
Total operating income (loss)	(41,623)	(54,364)	(51,691)
Total long-lived assets	656,629	674,866	656,172
United States
Geographic Information
Total revenue	330,003	360,881	292,250
Total operating income (loss)	(33,067)	(41,543)	(44,797)
Total long-lived assets	582,644	582,028	544,638
Canada
Geographic Information
Total revenue	21,818	16,241	13,922
Total operating income (loss)	(4,980)	(3,087)	(1,594)
Total long-lived assets	68,292	85,984	103,997
Other
Geographic Information
Total revenue	32,499	51,818	46,303
Total operating income (loss)	(3,576)	(9,734)	(5,300)
Total long-lived assets	$ 5,693	$ 6,854	$ 7,537

401(k) Plan (Details) - USD ($) $ in Thousands	12 Months Ended
401(k) Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 304	$ 1,040	$ 1,448

Supplier Concentrations (Details) - Natural gas expense - Supplier concentration	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Williams Gas Processing Company
Supplier Concentrations
Concentration risk	1.00%	4.00%	3.00%
Shell Energy
Supplier Concentrations
Concentration risk	16.00%	17.00%	22.00%
SoCal Gas Company And San Diego Gas And Electric
Supplier Concentrations
Concentration risk	7.00%	3.00%	7.00%
BP Energy
Supplier Concentrations
Concentration risk	19.00%	24.00%
Vermont Gas Company
Supplier Concentrations
Concentration risk	10.00%	0.00%

Capitalized Lease Obligation and Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Leases [Abstract]
Weighted average interest rate (as a percent)	9.71%
Future payments under these capital leases
2016	$ 1,688
2017	1,738
2018	1,739
2019	1,633
2020	660
Thereafter	633
Total minimum lease payments	8,091
Less amount representing interest	(1,643)
Present value of future minimum lease payments	6,448
Less current portion	(1,294)
Capital lease obligations, less current portion	5,154
Value of the equipment under capital lease	8,970	$ 4,898
Accumulated amortization	$ 2,766	$ 1,936

Capitalized Lease Obligation and Receivables - Future Receipts (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Leases [Abstract]
Sales-type leases interest rate (as a percent)	10.62%
Future receipts under the leases
2016	$ 563
2017	380
2018	251
2019	186
2020	186
Thereafter	1,798
Total	3,364
Less amount representing interest	(1,575)
Present value of future minimum lease receipts	$ 1,789

Fair Value Measurements (Details) - Fair value measured on recurring basis - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Warrants
Liabilities:
Derivative liability	$ 632	$ 1,416
Level 3 \| Warrants
Liabilities:
Derivative liability	632	1,416
Certificates of deposit
Assets:
Available-for-sale securities	48,551	35,355
Certificates of deposit \| Level 2
Assets:
Available-for-sale securities	48,551	35,355
Municipal bonds and notes
Assets:
Available-for-sale securities	16,790	38,652
Municipal bonds and notes \| Level 2
Assets:
Available-for-sale securities	16,790	38,652
Zero coupon bonds
Assets:
Available-for-sale securities	499	3,306
Zero coupon bonds \| Level 2
Assets:
Available-for-sale securities	499	3,306
Corporate bonds
Assets:
Available-for-sale securities	37,104	45,233
Corporate bonds \| Level 2
Assets:
Available-for-sale securities	$ 37,104	$ 45,233

Fair Value Measurements - Fair Value Recognition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other disclosures
Impairment of long-lived asset	$ 0	$ 4,772	$ 0
Series I Warrant Liabilities
Reconciliation of the beginning and ending balances of items measured at fair value using significant unobservable inputs (Level 3)
Beginning Balance	2,046	7,164
Gain included in earnings	(1,414)	(5,748)
Ending Balance	632	2,046	$ 7,164
Series I Warrant Liabilities \| NG Advantage
Reconciliation of the beginning and ending balances of items measured at fair value using significant unobservable inputs (Level 3)
Warrants acquired from NG Advantage acquisition	$ 0	$ 630

Fair Value Measurements - Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2015 $ / shares
Unobservable Input
Implied volatility of the Company's common stock	0.00%
Assumed discount rate	0.33%
Level 3 \| Series I Warrant Liabilities \| Black-Scholes Model
Unobservable Input
Current market price of the Company's common stock (in dollars per share)	$ 3.6
Exercise price of the warrant (in dollars per share)	$ 12.54
Dividend yield	0.00%
Remaining term of the warrant	3 months 29 days

Schedule II: Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowances for Doubtful Trade Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	$ 752	$ 832	$ 905
Charges (benefit) to operations	1,514	387	454
Deductions	(371)	(467)	(527)
Balance at the end of the period	1,895	752	832
Allowance for Doubtful Notes Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	2,850	2,416	909
Charges (benefit) to operations	1,142	890	1,507
Deductions	(2)	(456)	0
Balance at the end of the period	$ 3,990	$ 2,850	$ 2,416