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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2015	Jul. 29, 2015
Document and Entity Information
Entity Registrant Name	Clean Energy Fuels Corp.
Entity Central Index Key	0001368265
Document Type	10-Q
Document Period End Date	Jun. 30, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		90,560,410
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q2

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 53,296	$ 92,381
Restricted cash	4,469	6,012
Short-term investments	128,596	122,546
Accounts receivable, net of allowance for doubtful accounts of $752 and $970 as of December 31, 2014 and June 30, 2015, respectively	78,537	81,970
Other receivables	18,656	56,223
Inventories	31,347	34,696
Prepaid expenses and other current assets	15,172	19,811
Total current assets	330,073	413,639
Land, property and equipment, net	520,424	514,269
Notes receivable and other long-term assets, net	69,362	71,904
Investments in other entities	5,961	6,510
Goodwill	95,831	98,726
Intangible assets, net	49,310	55,361
Total assets	1,070,961	1,160,409
Current liabilities:
Current portion of long-term debt and capital lease obligations	5,977	4,846
Accounts payable	28,859	43,922
Accrued liabilities	51,327	56,760
Deferred revenue	5,566	14,683
Total current liabilities	91,729	120,211
Long-term debt and capital lease obligations, less current portion	504,769	500,824
Long-term debt, related party	65,000	65,000
Other long-term liabilities	7,971	9,339
Total liabilities	669,469	695,374
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.0001 par value. Authorized 1,000,000 shares; issued and outstanding no shares
Common stock, $0.0001 par value. Authorized 224,000,000 shares; issued and outstanding 90,203,344 shares and 90,527,189 shares at December 31, 2014 and June 30, 2015, respectively	9	9
Additional paid-in capital	904,058	898,106
Accumulated deficit	(518,533)	(457,441)
Accumulated other comprehensive loss	(10,878)	(3,248)
Total Clean Energy Fuels Corp. stockholders' equity	374,656	437,426
Noncontrolling interest in subsidiary	26,836	27,609
Total stockholders' equity	401,492	465,035
Total liabilities and stockholders' equity	$ 1,070,961	$ 1,160,409

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Condensed Consolidated Balance Sheets
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 970	$ 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	90,527,189	90,203,344
Common stock, outstanding shares	90,527,189	90,203,344

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenue:
Product revenues	$ 75,744	$ 86,473	$ 145,041	$ 172,262
Service revenues	11,124	11,660	27,675	21,146
Total revenues	86,868	98,133	172,716	193,408
Cost of sales (exclusive of depreciation and amortization shown separately below):
Product cost of sales	59,387	69,175	114,766	137,042
Service cost of sales	4,399	4,080	13,753	7,844
Derivative (gains) losses:
Series I warrant valuation	300	2,286	(583)	(2,169)
Selling, general and administrative	28,994	34,400	59,227	67,890
Depreciation and amortization	13,402	11,608	26,288	23,123
Total operating expenses	106,482	121,549	213,451	233,730
Operating loss	(19,614)	(23,416)	(40,735)	(40,322)
Interest expense, net	(9,973)	(10,130)	(19,868)	(19,640)
Other income (expense), net	317	1,121	864	(165)
Loss from equity method investments	(345)		(549)
Loss before income taxes	(29,615)	(32,425)	(60,288)	(60,127)
Income tax expense	(740)	(147)	(1,594)	(1,109)
Net loss	(30,355)	(32,572)	(61,882)	(61,236)
Loss from noncontrolling interest	393	266	773	337
Net loss attributable to Clean Energy Fuels Corp.	$ (29,962)	$ (32,306)	$ (61,109)	$ (60,899)
Loss per share attributable to Clean Energy Fuels Corp.:
Basic (in dollars per share)	$ (0.33)	$ (0.34)	$ (0.67)	$ (0.64)
Diluted (in dollars per share)	$ (0.33)	$ (0.34)	$ (0.67)	$ (0.64)
Weighted-average common shares outstanding:
Basic (in shares)	91,480,998	94,859,587	91,399,478	94,768,462
Diluted (in shares)	91,480,998	94,859,587	91,399,478	94,768,462

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net loss	$ (30,355)	$ (32,572)	$ (61,882)	$ (61,236)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	813	(442)	(4,868)	(246)
Foreign currency adjustments on intra-entity long-term investments	535	3,495	(2,776)	157
Unrealized (losses) gains on available-for sale securities	(2)	(81)	14	(399)
Total other comprehensive income (loss), net of tax	1,346	2,972	(7,630)	(488)
Comprehensive loss	(29,009)	(29,600)	(69,512)	(61,724)
Clean Energy Fuels Corp.
Net loss	(29,962)	(32,306)	(61,109)	(60,899)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	813	(442)	(4,868)	(246)
Foreign currency adjustments on intra-entity long-term investments	535	3,495	(2,776)	157
Unrealized (losses) gains on available-for sale securities	(2)	(81)	14	(399)
Total other comprehensive income (loss), net of tax	1,346	2,972	(7,630)	(488)
Comprehensive loss	(28,616)	(29,334)	(68,739)	(61,387)
Noncontrolling Interest
Net loss	(393)	(266)	(773)	(337)
Other comprehensive income (loss), net of tax:
Comprehensive loss	$ (393)	$ (266)	$ (773)	$ (337)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities:
Net loss	$ (61,882)	$ (61,236)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	26,288	23,123
Provision for doubtful accounts, notes and inventory	1,440	112
Loss on disposal of assets	42	8
Derivative loss	(583)	(2,169)
Stock-based compensation expense	5,353	6,398
Amortization of debt issuance cost	1,534	1,527
Accretion of notes payable	36	138
Equity method investment loss	549
Loss on contingent consideration for acquisition		101
Changes in operating assets and liabilities, net of assets and liabilities acquired and disposed:
Accounts and other receivables	40,636	(7,209)
Inventory	3,293	(5,382)
Prepaid expenses and other assets	7,329	1,891
Accounts payable	(11,570)	(1,715)
Accrued expenses and other	(15,323)	1,126
Net cash used in operating activities	(2,858)	(43,287)
Cash flows from investing activities:
Purchases of short-term investments	(71,739)	(70,868)
Maturities of short-term investments	64,861	55,144
Purchases and deposits on property and equipment	(26,236)	(61,840)
Loans made to customers	(2,510)	(2,295)
Payments on and proceeds from sales of loans receivable	892	2,240
Restricted cash	1,543	(3,846)
Net cash used in investing activities	(33,189)	(81,465)
Cash flows from financing activities:
Proceeds from issuance of common stock and exercise of stock options	599	684
Proceeds from debt instruments	6	12,720
Proceeds from revolving line of credit	24	23,621
Repayment of borrowings under revolving line of credit	(26)	(17,645)
Repayment of capital lease obligations and debt instruments	(2,509)	(9,338)
Payments for debt issuance costs		(914)
Net cash provided (used) by financing activities	(1,906)	9,128
Effect of exchange rates on cash and cash equivalents	(1,132)	291
Net decrease in cash	(39,085)	(115,333)
Cash, beginning of period	92,381	240,033
Cash, end of period	53,296	124,700
Supplemental disclosure of cash flow information:
Income taxes paid	497	690
Interest paid, net of approximately $1,992 and $534 capitalized, respectively	$ 18,184	$ 18,668

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Condensed Consolidated Statements of Cash Flows
Interest paid, capitalized	$ 534	$ 1,992

General	6 Months Ended
General	Jun. 30, 2015
General
General	Note 1—General Nature of Business: 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 otherwise) is engaged in the business of selling natural gas fueling solutions to its customers, primarily in the United States and Canada. The Company designs, builds, operates and maintains fueling stations and supplies customers with compressed natural gas (“CNG”) fuel for light, medium and heavy-duty vehicles and liquefied natural gas (“LNG”) fuel for medium and heavy-duty vehicles. The Company also manufactures, sells and services non-lubricated natural gas fueling compressors and other equipment used in CNG stations and LNG stations, provides operation and maintenance (“O&M”) services to customers, 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 renewable natural gas (“RNG”), which can be used as vehicle fuel or sold for renewable power generation, sells tradable credits generated by selling natural gas and RNG as a vehicle fuel, including credits generated under the California Low Carbon Fuel Standard (“LCFS Credits”) and Renewable Identification Numbers (“RIN Credits” or “RINs”) generated under the federal Renewable Fuel Standard Phase 2, helps customers acquire and finance natural gas vehicles and obtains local, state and federal grants and incentives. Basis of Presentation: The accompanying interim unaudited condensed 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 and cash flows as of and for the three and six months ended June 30, 2014 and 2015. All intercompany accounts and transactions have been eliminated in consolidation. The three and six month periods ended June 30, 2014 and 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015 or for any other interim period or for any future year. Certain information and disclosures normally included in the notes to the financial statements have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”), but the resultant disclosures contained herein are in accordance with accounting principles generally accepted in the United States of America (“US GAAP”) as they apply to interim reporting. The condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2014 that are included in the Company’s Annual Report on Form 10-K filed with the SEC on February 26, 2015. Use of Estimates: The preparation of condensed 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 condensed consolidated financial statements and revenues and expenses recorded during the reporting period. Actual results could differ from those estimates. Significant estimates made in preparing the condensed consolidated financial statements include (but are not limited to) those related to revenue recognition, warranty reserves, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, and stock-based compensation expense.

Acquisition and Divestitures

6 Months Ended

Jun. 30, 2015

Acquisition and Divestitures

Acquisition and Divestitures

Note 2— Acquisition and Divestitures

NG Advantage

On October 14, 2014, the Company entered 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 payable by the Company in two payments as follows: (i) $3,000 was paid January 13, 2015 and (ii) the remaining $15,650 was paid 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 condensed consolidated financial statements.

The Company accounted for this acquisition in accordance with Financial Accounting Standards Board (“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 preliminary 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		)
Non-controlling 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 cancelled.

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 fully deductible for income tax purposes.

Management determined the fair value of the non-controlling 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 non-controlling 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 renewable natural gas 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 and $40,588 in cash in December 2014 and may receive up to an additional $1,250 in cash, subject to the results of certain performance tests to be performed at the McCommas Bluff project in accordance the terms of the Agreement. The Company will continue 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 gain of $11,998 that was recorded in the line item gain from the sale of subsidiary in the Company’s consolidated statement of operations for the year ended December 31, 2014. Included in the determination of the gain was goodwill of $7,205 that was allocated to the transaction in accordance with FASB authoritative guidance for subsequent measure of goodwill which requires the Company to allocate goodwill 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, in accordance with FASB authoritative guidance, 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.

Cash and Cash Equivalents	6 Months Ended
Cash and Cash Equivalents	Jun. 30, 2015
Cash and Cash Equivalents
Cash and Cash Equivalents	Note 3—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. 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, CDIC, and other foreign insurance limits were approximately $88,740 and $50,117 as of December 31, 2014 and June 30, 2015, respectively.

- Definition

The entire disclosure for cash and cash equivalent footnotes, which may include the types of deposits and money market instruments, applicable carrying amounts, restricted amounts and compensating balance arrangements. Cash and equivalents include: (1) currency on hand (2) demand deposits with banks or financial institutions (3) other kinds of accounts that have the general characteristics of demand deposits (4) short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments maturing within three months from the date of acquisition qualify.

+ References + Details

Restricted Cash

6 Months Ended

Jun. 30, 2015

Restricted Cash.

Restricted Cash

Note 4—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 June 30, 2015:

	December 31, 2014		June 30, 2015
Short-term restricted cash:
Standby letters of credit	$	1,753	$	1,753
Canton bonds — current operating costs	4,259		2,716

Total short-term restricted cash	$	6,012	$	4,469

Investments

6 Months Ended

Jun. 30, 2015

Investments

Investments

Note 5—Investments

Available-for-sale investments are carried at fair value, inclusive of unrealized gains and losses. Net 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 June 30, 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

	$	122,605	$	(59	)	$	122,546

Short-term investments as of June 30, 2015 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains/(Losses)			Estimated Fair Value
Municipal bonds & notes	$	38,825	$	(24	)	$	38,801
Zero coupon bonds	5,013		3			5,016
Corporate bonds	38,850		(24		)	38,826
Certificate of deposits	45,953		—			45,953

	$	128,641	$	(45	)	$	128,596

- Definition

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

+ References + Details

Other Receivables

6 Months Ended

Jun. 30, 2015

Other Receivables

Other Receivables

Note 6—Other Receivables

Other receivables at December 31, 2014 and June 30, 2015 consisted of the following:

	December 31, 2014		June 30, 2015
Loans to customers to finance vehicle purchases	$	8,257	$	8,982
Accrued customer billings	10,143		5,730
Fuel tax and carbon credits	34,250		155
Other	3,573		3,789

	$	56,223	$	18,656

Inventories

6 Months Ended

Jun. 30, 2015

Inventories

Inventories

Note 7—Inventories

Inventory consists of 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 June 30, 2015:

	December 31, 2014		June 30, 2015
Raw materials and spare parts	$	31,389	$	28,080
Work in process	3,292		1,879
Finished goods	15		1,388

	$	34,696	$	31,347

Land, Property and Equipment

6 Months Ended

Jun. 30, 2015

Land, Property and Equipment

Land, Property and Equipment

Note 8—Land, Property and Equipment

Land, property and equipment at December 31, 2014 and June 30, 2015 are summarized as follows:

	December 31, 2014			June 30, 2015
Land	$	2,858		$	2,858
LNG liquefaction plants	94,636			94,636
RNG plants	45,359			45,451
Station equipment	265,086			293,851
Trailers	40,067			50,355
Other equipment	74,796			79,929
Construction in progress	163,737			146,431

	686,539			713,511
Less: accumulated depreciation	(172,270		)	(193,087		)

	$	514,269		$	520,424

Included in land, property and equipment are capitalized software costs of $21,004 and $21,558 as of December 31, 2014 and June 30, 2015, respectively. The accumulated amortization on the capitalized software costs is $10,740 and $12,287 as of December 31, 2014 and June 30, 2015, respectively. The Company recorded $843 and $831 of amortization expense related to the capitalized software costs during the three months ended June 30, 2014 and June 30, 2015, respectively. For the six month periods ended June 30, 2014 and 2015, the Company recorded $1,611 and $1,547 of amortization expense related to the capitalized software costs respectively.

As of June 30, 2014 and June 30, 2015, $16,360 and $9,720, respectively are included in accounts payable and accrued liabilities balances, which amounts are related to purchases of property and equipment. These amounts are excluded from the condensed consolidated statements of cash flows as they are non-cash investing activities.

Investments in Other Entities	6 Months Ended
Investments in Other Entities	Jun. 30, 2015
Investments in Other Entities
Investments in Other Entities	Note 9—Investments in Other Entities On September 16, 2014, the Company formed a joint venture with Mansfield Ventures LLC (“Mansfield”) 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 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 $345 and $549 for the three and six months ended June 30, 2015, respectively and has an investment balance of $4,961 at June 30, 2015.

Accrued Liabilities

6 Months Ended

Jun. 30, 2015

Accrued Liabilities

Accrued Liabilities

Note 10—Accrued Liabilities

Accrued liabilities at December 31, 2014 and June 30, 2015 consisted of the following:

	December 31, 2014		June 30, 2015
Salaries and wages	$	9,041	$	8,206
Accrued gas and equipment purchases	12,340		15,904
Accrued property and other taxes	5,178		7,199
Accrued professional fees	1,084		742
Accrued employee benefits	3,208		4,125
Accrued warranty liability	2,302		2,423
Accrued interest	3,748		3,721
Other	19,859		9,007

	$	56,760	$	51,327

Warranty Liability

6 Months Ended

Jun. 30, 2015

Warranty Liability

Warranty Liability

Note 11—Warranty Liability

The Company records warranty liabilities at the time of sale for the estimated costs that may be incurred under its standard warranty. Changes in the warranty liability are presented in the following table:

	June 30, 2014			June 30, 2015
Warranty liability at beginning of year	$	2,545		$	2,302
Costs accrued for new warranty contracts and changes in estimates for pre-existing warranties	2,455			1,220
Service obligations honored	(1,685		)	(1,099		)

Warranty liability at end of period	$	3,315		$	2,423

Long-Term Debt

6 Months Ended

Jun. 30, 2015

Long-Term Debt

Long-Term Debt

Note 12—Long-Term Debt

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 (the “CHK Financing”) pursuant to the issuance of three convertible promissory notes, 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 change in 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 cancelled the existing CHK Notes and re-issued replacement notes, and the Company also issued notes to the Buyers in exchange for the June Advance (the re-issued 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 the 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 a 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 the its common stock if, following the second anniversary of the issuance of a 7.5% Note, such shares trade at a 40% premium to the 7.5% Notes Conversion Price for at least 20 trading days in any consecutive 30 trading day period. The entire principal balance of each 7.5% Note is due and payable seven years following its issuance and the Company may repay each 7.5% Note in shares of its common stock or cash. All of the shares issuable upon exercise 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. No events of default under the 7.5% Notes had occurred as of June 30, 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, (i) Mr. 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, and (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.

At June 30, 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.

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 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 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 each Purchaser’s option 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 exercise 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 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 June 30, 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.

GE Loans

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”).

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 will terminate if the initial loans under the Credit Agreement (collectively, “Loans” and, with respect to each Project “Tranche A Loans” and “Tranche B Loans”) for the Projects are not made prior to December 31, 2016 (rather than December 31, 2014, as the Credit Agreement originally provided), (ii) each Project must 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) (with respect to each Project, the “Date Certain”), and (iii) prior to the funding of the Loans, the Borrowers will be required to enter into agreements with GE Oil & Gas, Inc. relating to the purchase of equipment for the Projects.

The Borrowers’ ability to obtain the Loans under the Credit Agreement is subject to the satisfaction of certain conditions, including each of the (i) acquisition of title to, or leasehold interests in, the sites upon which the Projects will be constructed, (ii) receipt of all governmental approvals necessary in connection with the design, development, ownership, construction, installation, operation and maintenance of the Projects, and (iii) commitment of all utility services necessary for the construction and operation of the Projects.

The Credit Agreement further provides that (i) the then existing Loans with respect to each Project must be converted into term loans with eight year amortization schedules (“Term Loans”) on or before the Date Certain with respect to such Project (the date of such conversion with respect to each Project, the “Conversion Date”), provided that if such Loans are not converted into Term Loans by the applicable Date Certain, such Loans must be repaid by the applicable Date Certain, (ii) each Term Loan will be due and payable on the eighth anniversary of the Conversion Date with respect to such Term Loan, and (iii) at any time prior to the applicable Conversion Date, the Loans may be prepaid in whole, and at any time after the applicable Conversion Date, the Loans may be prepaid in whole or in part. The Company expects the Loans to bear interest at an annual rate equal to the then- current LIBOR rate plus 7.00%, provided that for purposes of the Credit Agreement, the then-current LIBOR rate will always be at least 1.00%. The Credit Agreement includes various customary covenants, including debt service coverage ratios and a commitment fee on the unutilized loan amounts of 0.5% per annum, and also provides for customary events of default which, if such events occur, would permit or require the Loans to become or to be declared due and payable. As of June 30, 2015, the Company has not drawn any money under the Credit Agreement and was in compliance with the financial covenants. The commitment fee, which is charged to interest expense in the condensed consolidated statements of operations, was $253 for each of the three months ended June 30, 2014 and 2015, respectively. For the six month periods ended June 30, 2014 and 2015, the Company recorded $503 of interest expense for each period related to the commitment fee.

The Loans are secured by (i) a first priority security interest in all of the Borrowers’ assets, including the Projects, and (ii) a pledge of the Borrowers’ outstanding ownership interests. In addition, the Company has executed a guaranty in favor of GE (“Guaranty”), pursuant to which the Company has guaranteed all of the Borrowers’ obligations under the Credit Agreement, including repayment of all Loans.

The Company and GE also entered an equity contribution agreement (the “EC Agreement”) pursuant to which the Company agreed to pay at least 25% of the budgeted cost of the Projects and all additional costs that exceed such expected budgeted costs, in each case, in the form of equity contributions to the Borrowers (“Equity Contributions”). The EC Agreement also requires the Company to provide, concurrent with GE’s extension of the initial Loans under the Credit Agreement, letter(s) of credit in an amount equal to the Company’s then-current unfunded Equity Contributions.

Concurrently with the execution of the Credit Agreement, the Company issued to GE a warrant to purchase up to five million shares of its common stock (see note 13).

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 Notes (which is equivalent to an initial conversion price of approximately $15.60 per share of the Company’s common stock). The conversion rate is subject to adjustment upon the occurrence of certain specified events as described in the Indenture.

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

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

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

The Indenture contains customary events of default with customary cure periods, including, without limitation, failure to make required payments or deliveries of shares of the Company’s common stock when due under the Indenture, failure to comply with certain covenants under the Indenture, failure to pay when due or acceleration of certain other indebtedness of the Company or certain of its subsidiaries, and certain events of bankruptcy and insolvency of the Company or certain of its subsidiaries. The occurrence of an event of default under the Indenture will allow either the trustee or the holders of at least 25% in principal amount of the then-outstanding 5.25% Notes to accelerate, or upon an event of default arising from certain events of bankruptcy or insolvency of the Company, will automatically cause the acceleration of, all amounts due under the 5.25% Notes. No events of default under the 5.25% Notes had occurred as of June 30, 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.

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 “Bonds”).

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

Canton used the 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 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 Bonds that was entered 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 under the Loan Agreement had occurred as of June 30, 2015.

Other Debt

The Company has other debt due at various dates through 2020 bearing interest at rates up to 15.49% and with a weighted average interest rate of 6.80% and 6.67% as of December 31, 2014 and June 30, 2015, respectively.

Long-term debt and capital lease obligations at December 31, 2014 and June 30, 2015 consisted of the following:

	December 31, 2014			June 30, 2015
7.5% Notes	$	150,000		$	150,000
SLG Notes	145,000			145,000
5.25% Notes	250,000			250,000
Canton Bonds ($11,400 non-recourse to the Company)	12,150			11,600
Capital lease obligations	2,692			7,754
Other debt	10,828			11,392

Total debt and capital lease obligations	570,670			575,746
Less amounts due within one year	(4,846		)	(5,977		)

Total long-term debt and capital lease obligations	$	565,824		$	569,769

Net Loss Per Share

6 Months Ended

Jun. 30, 2015

Net Loss Per Share

Net Loss Per Share

Note 13—Net Loss Per Share

Basic net loss per share is based upon the weighted-average number of shares outstanding and issuable for little or no cash consideration during each period. Diluted net loss per share reflects the impact of the assumed exercise of dilutive stock options and warrants. On September 11, 2014, the Company determined it no longer met certain conditions required to include in its weighted average share calculations 4,000,000 of the 5,000,000 shares of common stock issuable upon exercise of a warrant issued to GE concurrently with the execution of the Credit Agreement (the “GE Warrant”). As a result, since September 11, 2014, the Company has (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 earnings 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 earnings per share calculations, as 500,000 shares issuable upon exercise of the GE Warrant were exercisable as of the execution of the Credit Agreement and an additional 500,000 shares issuable upon exercise of the GE Warrant became exercisable on December 31, 2014. The information required to compute basic and diluted earnings per share is as follows:

	Three Months Ended June 30,		Six Months Ended June 30,
	2014	2015	2014	2015
Basic and diluted:
Weighted average number of common shares outstanding	94,859,587	91,480,998	94,768,462	91,399,478

Certain securities were excluded from the diluted earnings per share calculations for the three and six months ended June 30, 2014 and 2015, respectively, as the inclusion of the securities would be anti-dilutive to the calculations. The amounts of these securities that were outstanding as of June 30, 2014 and 2015 for these instruments are as follows:

	June 30,
	2014	2015
Options	11,858,681	10,782,668
Warrants	2,130,682	6,130,682
Convertible Notes	35,185,979	35,185,979
Restricted Stock Units	2,080,336	2,972,252

Stock-Based Compensation

6 Months Ended

Jun. 30, 2015

Stock-Based Compensation

Stock-Based Compensation

Note 14—Stock-Based Compensation

The following table summarizes the compensation expense and related income tax benefit related to the stock-based compensation expense recognized during the periods:

	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2015		2014		2015

Stock-based compensation expense	$	2,978	$	2,663	$	6,398	$	5,353

Stock-based compensation expense, net of tax	$	2,978	$	2,663	$	6,398	$	5,353

Stock Options

The following table summarizes the Company’s stock option activity during the six months ended June 30, 2015:

	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding, December 31, 2014	11,486,301		$	11.91
Options granted	465,000		6.15
Options exercised	(607,500	)	2.96
Options forfeited	(561,133	)	14.13

Outstanding, June 30, 2015	10,782,668		$	12.05	5.01	$	—

Exercisable, June 30, 2015	9,340,463		$	12.48	4.43	$	—

As of June 30, 2015, there was $4,979 of total unrecognized compensation cost related to non-vested shares underlying outstanding stock options. That cost is expected to be recognized over a weighted average period of 1.6 years. The total fair value of shares vested during the six months ended June 30, 2015 was $4,314.

The Company is obligated to issue shares of its common stock upon the exercise of outstanding stock options. The intrinsic value of all stock options exercised during the six months ended June 30, 2014 and 2015 was $1,419 and $2,196, respectively.

The fair value of each stock option granted during the six months ended June 30, 2015 was estimated as of the date of grant using the Black-Scholes option pricing model and the following assumptions:

		Six Months Ended June 30, 2015
Dividend yield		0.00%
Expected volatility		59.2% to 61.5%
Risk-free interest rate		1.7% to 1.8%
Expected life in years		6.0

The weighted-average grant date fair values of stock options granted during the six months ended June 30, 2014 and 2015 was $5.99 and $3.42, respectively. The volatility amounts used during these periods were estimated based on the Company’s historical volatility and the Company’s implied volatility of its traded options for such periods. 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 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 $3,819 and $2,843 of stock option expense during the six months ended June 30, 2014 and 2015, respectively. The Company has not recorded any tax benefit related to its stock option expense.

Market-Based Restricted Stock Units

The Company issued market-based 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 (i) 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 (ii) 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 will be automatically forfeited. The Market-Based RSUs are subject to the terms and conditions of the Company’s Amended and Restated 2006 Equity Incentive Plan (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 during the six months ended June 30, 2015:

	Number of Shares	Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
Outstanding, December 31, 2014	1,769,000	$	10.67
RSUs granted	—	—

Outstanding and non-vested, June 30, 2015	1,769,000	$	10.67	1.1

As of June 30, 2015, there was $1,180 of total unrecognized compensation cost related to non-vested shares underlying outstanding Market-Based RSUs. That cost is expected to be recognized over a weighted average period of 0.6 year.

The Company recorded $1,775 and $885 of expense during the six months ended June 30, 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 issued service-based restricted stock units (“Service-Based RSUs”) to certain employees that vest annually over the three years following the date of issuance 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 Company’s 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 during the six months ended June 30, 2015:

	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
Non-vested at December 31, 2014	822,752		$	5.82
RSUs granted	419,500		6.05
RSUs vested	—		—
RSUs forfeited	(39,000	)	5.58

Outstanding and non-vested at June 30, 2015	1,203,252		$	5.91	2.5

As of June 30, 2015, there was $5,643 of total unrecognized compensation cost related to non-vested shares underlying outstanding Service-Based RSUs. That cost is expected to be recognized evenly over a period of 2.4 years.

The Company recorded $153 and $1,050 of expense during the six months ended June 30, 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 three months ended June 30, 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.

The Company recorded $37 and $25 of expense related to the ESPP during the six months ended June 30, 2014 and 2015, respectively. The Company has not recorded any tax benefits related to its ESPP expense. At June 30, 2015, the Company had sold an aggregate of 105,257 shares pursuant to the ESPP.

Non-Qualified Non-Public Subsidiary Unit Options

In September 2013, the Company’s wholly owned subsidiary, Clean Energy Renewable Fuels, LLC (“Clean Energy Renewables”, formerly known as CERF), adopted the Clean Energy Renewable Fuels, LLC 2013 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 six months ended June 30, 2015:

	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding, December 31, 2014	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited	(7,000	)	40.80

Outstanding and non-vested, June 30, 2015	108,000		$	40.80	8.22	$	—

As of June 30, 2015, there was $1,384 of total unrecognized compensation cost related to non-vested units underlying outstanding options issued pursuant to the Clean Energy Renewables Plan. That cost is expected to be recognized over a weighted average period of 0.9 years.

Clean Energy Renewables recorded a total of $614 and $538 of unit option expense during the six months ended June 30, 2014 and 2015, respectively. Clean Energy Renewables has not recorded any tax benefit related to its unit option expense.

In March 2013, NG Advantage, adopted the NG Advantage LLC 2013 Unit Option Plan (the “NG Advantage Plan”). NG Advantage recorded a total of $12 of unit option expense during the six months ended June 30, 2015.

Environmental Matters, Litigation, Claims, Commitments and Contingencies	6 Months Ended
	Jun. 30, 2015
Environmental Matters, Litigation, Claims, Commitments and Contingencies
Environmental Matters, Litigation, Claims, Commitments and Contingencies	Note 15—Environmental Matters, Litigation, Claims, Commitments and Contingencies 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 which would have a material impact on the Company’s condensed 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. 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 such 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 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.

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2015
Income Taxes
Income Taxes	Note 16—Income Taxes The Company’s income tax provision for the three and six months ended June 30, 2015 was $740 and $1,594, respectively. The tax expense for the three and six months ended June 30, 2014 was $147 and $1,109 respectively. Tax expense for all periods was comprised of taxes due on the Company’s U.S. and foreign operations. The increase in the Company’s income tax provision for the six months ended June 30, 2015 as compared to the tax provision for the six months ended June 30, 2014 was primarily attributed to increase in the earnings of foreign subsidiaries. The effective tax rate for the three months and six months ended June 30, 2014 and 2015 are different from the federal statutory tax rate primarily as a result of losses for which no tax benefit has been recognized. The Company did not record a change in its liability for unrecognized tax benefits or penalties in the three and six months ended June 30, 2014 or June 30, 2015, and the net interest incurred was immaterial for such periods.

Fair Value Measurements

6 Months Ended

Jun. 30, 2015

Fair Value Measurements

Fair Value Measurements

Note 17—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 six months ended June 30, 2015, the Company’s financial instruments consisted of available-for-sale securities, debt instruments, and its Series I warrants. For available-for-sale securities, the fair values are determined by the most recent quoted prices for identical assets in markets that are not active and quoted prices for similar assets in active markets, and thus represent Level 2 fair value measurements. The fair values of the Company’s debt instruments approximated their carrying values at December 31, 2014 and June 30, 2015. The Company uses the Black-Scholes model to value the Series I warrants. The Company believes the best method to approximate a market participant’s view of the volatility of its Series I warrants has been to use the implied volatilities of its short-term (i.e. 3 to 9 month) traded options and extrapolate the data over the remaining term of the Series I warrants, which was approximately 0.8 years as of June 30, 2015. This method has been utilized consistently in the periods presented. Given that the extrapolation beyond the term of the short-term exchange traded options is not based on observable market inputs for a significant portion of the remaining term of the warrants, the Series I warrants have been classified as a Level 3 fair value measurement.

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 June 30, 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:
Series I warrants(2)	1,416		—		—		1,416

Description	Balance at June 30, 2015		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Certificate of deposits	$	45,953	$	—	$	45,953	$	—
Municipal bonds and notes	38,801		—		38,801		—
Zero coupon bonds	5,016				5,016
Corporate bonds	38,826		—		38,826		—
Liabilities:
Series I warrants(2)	833		—		—		833

(1) Included in short-term investments in the condensed consolidated balance sheets. See note 5 for further information.

(2) Included in accrued liabilities in the condensed 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 table above that used significant unobservable inputs (Level 3).

Liabilities: Series I Warrants	June 30, 2014			June 30, 2015
Beginning Balance	$	7,164		$	1,416
Total gain included in earnings	(2,169		)	(583		)

Ending Balance	$	4,995		$	833

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

Fair value measurements of liabilities, which fall within Level 3 of the fair value hierarchy, are determined by the Company’s accounting department, who report to the Company’s Chief Financial Officer. 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 model based on the following inputs as of June 30, 2015:

Unobservable Input		Range or Weighted Average
Current market price of the Company’s common stock		$5.62
Exercise price of the warrant		$12.68
Dividend yield		0.00%
Remaining term of the warrant		0.83
Implied volatility of the Company’s common stock		80.36%
Assumed discount rate		Simple average of 0.28%

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 warrants 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 Series I warrants due to the dividend assumption.

Non-Financial Assets

No impairments of long-lived assets measured at fair value on a non-recurring basis have been incurred during the six months ended June 30, 2014 and 2015. The Company’s use of these non-financial assets does not differ from their highest and best use as determined from the perspective of a market participant.

- 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

Recently Adopted Accounting Changes and Recently Issued Accounting Standards	6 Months Ended
	Jun. 30, 2015
Recently Adopted Accounting Changes and Recently Issued Accounting Standards
Recently Adopted Accounting Changes and Recently Issued Accounting Standards	Note 18—Recently Adopted Accounting Changes and Recently Issued Accounting Standards In May 2014, the FASB issued ASU No. 2014-09, 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 guidance is effective for annual and interim reporting periods beginning after December 15, 2016, with early adoption prohibited. The FASB decided on July 9, 2015 to defer the effective date of the new revenue standard by one year. As a result, this standard will be effective for fiscal years, and interim reporting periods within those years, beginning after December 15, 2017. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements. 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. Earlier adoption is permitted but the Company does not anticipate electing early adoption. As of June 30, 2015, the Company has $4,840 of debt issuance costs.

- Definition

The entire disclosure of changes in accounting principles, including adoption of new accounting pronouncements, that describes the new methods, amount and effects on financial statement line items.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Change in Accounting Principle
-URI http://asc.fasb.org/extlink&oid=6507316

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Direct Effects of a Change in Accounting Principle
-URI http://asc.fasb.org/extlink&oid=6510796

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=56949863&loc=d3e22583-107794

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Indirect Effects of a Change in Accounting Principle
-URI http://asc.fasb.org/extlink&oid=6515603

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=56949863&loc=d3e22580-107794

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.10-01.(b)(6))
-URI http://asc.fasb.org/extlink&oid=27015980&loc=d3e46468-122699

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Accounting Change
-URI http://asc.fasb.org/extlink&oid=6503790

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=56949863&loc=d3e22499-107794

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Retrospective Application
-URI http://asc.fasb.org/extlink&oid=6523989

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=51655806&loc=d3e765-108305

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=51655806&loc=d3e725-108305

+ Details

Alternative Fuels Excise Tax Credit	6 Months Ended
Alternative Fuels Excise Tax Credit	Jun. 30, 2015
Alternative Fuels Excise Tax Credit
Alternative Fuels Excise Tax Credit	Note 19—Alternative Fuels Excise Tax Credit From October 1, 2006 through December 31, 2014, the Company was eligible to receive a federal fuel tax credit (“VETC”) of $0.50 per gasoline gallon equivalent of CNG and $0.50 per liquid gallon of LNG that it sold as vehicle fuel. Based on the service relationship with its customers, either the Company or its customers claimed the credit. The Company records its VETC credits as revenue in its condensed consolidated statements of operations, as the credits are fully refundable and do not need to offset income tax liabilities to be received. VETC revenues recognized in December 2014 for the 2014 calendar year were $28,359. The Company did not recognize any VETC revenue during the three or six months ended June 30, 2015, as the program under which the Company received such credit expired December 31, 2014 and has not been renewed.

General (Policies)	6 Months Ended
General (Policies)	Jun. 30, 2015
General
Basis of Presentation	Basis of Presentation: The accompanying interim unaudited condensed 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 and cash flows as of and for the three and six months ended June 30, 2014 and 2015. All intercompany accounts and transactions have been eliminated in consolidation. The three and six month periods ended June 30, 2014 and 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015 or for any other interim period or for any future year. Certain information and disclosures normally included in the notes to the financial statements have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”), but the resultant disclosures contained herein are in accordance with accounting principles generally accepted in the United States of America (“US GAAP”) as they apply to interim reporting. The condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2014 that are included in the Company’s Annual Report on Form 10-K filed with the SEC on February 26, 2015.
Use of Estimates	Use of Estimates: The preparation of condensed 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 condensed consolidated financial statements and revenues and expenses recorded during the reporting period. Actual results could differ from those estimates. Significant estimates made in preparing the condensed consolidated financial statements include (but are not limited to) those related to revenue recognition, warranty reserves, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, and stock-based compensation expense.

Acquisition and Divestitures (Tables)

6 Months Ended

Jun. 30, 2015

NG Advantage

Acquisition and Divestitures

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

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		)
Non-controlling interest	(28,075		)

Total purchase price	$	37,650

Restricted Cash (Tables)

6 Months Ended

Jun. 30, 2015

Restricted Cash.

Schedule of components of restricted cash

	December 31, 2014		June 30, 2015
Short-term restricted cash:
Standby letters of credit	$	1,753	$	1,753
Canton bonds — current operating costs	4,259		2,716

Total short-term restricted cash	$	6,012	$	4,469

Investments (Tables)

6 Months Ended

Jun. 30, 2015

Investments

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

	$	122,605	$	(59	)	$	122,546

Short-term investments as of June 30, 2015 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains/(Losses)			Estimated Fair Value
Municipal bonds & notes	$	38,825	$	(24	)	$	38,801
Zero coupon bonds	5,013		3			5,016
Corporate bonds	38,850		(24		)	38,826
Certificate of deposits	45,953		—			45,953

	$	128,641	$	(45	)	$	128,596

Other Receivables (Tables)

6 Months Ended

Jun. 30, 2015

Other Receivables

Schedule of other receivables

	December 31, 2014		June 30, 2015
Loans to customers to finance vehicle purchases	$	8,257	$	8,982
Accrued customer billings	10,143		5,730
Fuel tax and carbon credits	34,250		155
Other	3,573		3,789

	$	56,223	$	18,656

Inventories (Tables)

6 Months Ended

Jun. 30, 2015

Inventories

Schedule of inventories

	December 31, 2014		June 30, 2015
Raw materials and spare parts	$	31,389	$	28,080
Work in process	3,292		1,879
Finished goods	15		1,388

	$	34,696	$	31,347

Land, Property and Equipment (Tables)

6 Months Ended

Jun. 30, 2015

Land, Property and Equipment

Summary of land, property and equipment

	December 31, 2014			June 30, 2015
Land	$	2,858		$	2,858
LNG liquefaction plants	94,636			94,636
RNG plants	45,359			45,451
Station equipment	265,086			293,851
Trailers	40,067			50,355
Other equipment	74,796			79,929
Construction in progress	163,737			146,431

	686,539			713,511
Less: accumulated depreciation	(172,270		)	(193,087		)

	$	514,269		$	520,424

Accrued Liabilities (Tables)

6 Months Ended

Jun. 30, 2015

Accrued Liabilities

Schedule of accrued liabilities

	December 31, 2014		June 30, 2015
Salaries and wages	$	9,041	$	8,206
Accrued gas and equipment purchases	12,340		15,904
Accrued property and other taxes	5,178		7,199
Accrued professional fees	1,084		742
Accrued employee benefits	3,208		4,125
Accrued warranty liability	2,302		2,423
Accrued interest	3,748		3,721
Other	19,859		9,007

	$	56,760	$	51,327

Warranty Liability (Tables)

6 Months Ended

Jun. 30, 2015

Warranty Liability

Schedule of changes in the warranty liability

	June 30, 2014			June 30, 2015
Warranty liability at beginning of year	$	2,545		$	2,302
Costs accrued for new warranty contracts and changes in estimates for pre-existing warranties	2,455			1,220
Service obligations honored	(1,685		)	(1,099		)

Warranty liability at end of period	$	3,315		$	2,423

Long-Term Debt (Tables)

6 Months Ended

Jun. 30, 2015

Long-Term Debt

Schedule of long-term debt and capital lease obligations

	December 31, 2014			June 30, 2015
7.5% Notes	$	150,000		$	150,000
SLG Notes	145,000			145,000
5.25% Notes	250,000			250,000
Canton Bonds ($11,400 non-recourse to the Company)	12,150			11,600
Capital lease obligations	2,692			7,754
Other debt	10,828			11,392

Total debt and capital lease obligations	570,670			575,746
Less amounts due within one year	(4,846		)	(5,977		)

Total long-term debt and capital lease obligations	$	565,824		$	569,769

Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2015

Net Loss Per Share

Schedule of information required to compute basic and diluted earnings per share

	Three Months Ended June 30,		Six Months Ended June 30,
	2014	2015	2014	2015
Basic and diluted:
Weighted average number of common shares outstanding	94,859,587	91,480,998	94,768,462	91,399,478

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

	June 30,
	2014	2015
Options	11,858,681	10,782,668
Warrants	2,130,682	6,130,682
Convertible Notes	35,185,979	35,185,979
Restricted Stock Units	2,080,336	2,972,252

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2015

Stock-based compensation

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2015		2014		2015

Stock-based compensation expense	$	2,978	$	2,663	$	6,398	$	5,353

Stock-based compensation expense, net of tax	$	2,978	$	2,663	$	6,398	$	5,353

Stock Options

Stock-based compensation

Summary of option activity

	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding, December 31, 2014	11,486,301		$	11.91
Options granted	465,000		6.15
Options exercised	(607,500	)	2.96
Options forfeited	(561,133	)	14.13

Outstanding, June 30, 2015	10,782,668		$	12.05	5.01	$	—

Exercisable, June 30, 2015	9,340,463		$	12.48	4.43	$	—

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

		Six Months Ended June 30, 2015
Dividend yield		0.00%
Expected volatility		59.2% to 61.5%
Risk-free interest rate		1.7% to 1.8%
Expected life in years		6.0

Stock Options | Clean Energy Renewables Plan

Stock-based compensation

Summary of option activity

	Number of Units		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding, December 31, 2014	115,000		$	40.80
Options granted	—		—
Options exercised	—		—
Options forfeited	(7,000	)	40.80

Outstanding and non-vested, June 30, 2015	108,000		$	40.80	8.22	$	—

Market-Based RSUs

Stock-based compensation

Summary of RSU activity

	Number of Shares	Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
Outstanding, December 31, 2014	1,769,000	$	10.67
RSUs granted	—	—

Outstanding and non-vested, June 30, 2015	1,769,000	$	10.67	1.1

Service-Based RSUs

Stock-based compensation

Summary of RSU activity

	Number of Shares		Weighted Average Fair Value at Grant Date		Weighted Average Remaining Contractual Term (in years)
Non-vested at December 31, 2014	822,752		$	5.82
RSUs granted	419,500		6.05
RSUs vested	—		—
RSUs forfeited	(39,000	)	5.58

Outstanding and non-vested at June 30, 2015	1,203,252		$	5.91	2.5

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2015

Fair Value Measurements

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

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:
Series I warrants(2)	1,416		—		—		1,416

Description	Balance at June 30, 2015		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Certificate of deposits	$	45,953	$	—	$	45,953	$	—
Municipal bonds and notes	38,801		—		38,801		—
Zero coupon bonds	5,016				5,016
Corporate bonds	38,826		—		38,826		—
Liabilities:
Series I warrants(2)	833		—		—		833

(1) Included in short-term investments in the condensed consolidated balance sheets. See note 5 for further information.

(2) Included in accrued liabilities in the condensed 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: Series I Warrants	June 30, 2014			June 30, 2015
Beginning Balance	$	7,164		$	1,416
Total gain included in earnings	(2,169		)	(583		)

Ending Balance	$	4,995		$	833

Level 3 | Series I Warrant Liabilities

Fair Value Measurements

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

Unobservable Input		Range or Weighted Average
Current market price of the Company’s common stock		$5.62
Exercise price of the warrant		$12.68
Dividend yield		0.00%
Remaining term of the warrant		0.83
Implied volatility of the Company’s common stock		80.36%
Assumed discount rate		Simple average of 0.28%

Acquisition and Divestitures (Details) $ in Thousands	Oct. 14, 2014 USD ($) item	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Allocation of aggregate purchase price
Goodwill		$ 95,831	$ 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)
Non-controlling 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
Acquisition 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 \| item	2
NG Advantage \| Due by January 2015 \| Common unit purchase agreement
Acquisition and Divestitures
Consideration liability	$ 3,000
NG Advantage \| Due by April 2015 \| Common unit purchase agreement
Acquisition and Divestitures
Consideration liability	$ 15,650

Acquisition and Divestitures (Details 2) - USD ($) $ in Thousands				1 Months Ended
	Dec. 29, 2014	Sep. 04, 2014	Sep. 03, 2014	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2015
Divestitures
Goodwill				$ 98,726		$ 95,831
Dallas Clean Energy, LLC \| Mavrix
Divestitures
Ownership interest before transaction (as a percent)			70.00%
Dallas Clean Energy, LLC \| Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Cambrian
Divestitures
Cash consideration received on sale of subsidiary				$ 40,588	$ 6,992
Additional consideration that may be received under terms of the agreement	$ 1,250
Gain from sale of subsidiary	11,998
Goodwill	$ 7,205
Dallas Clean Energy, LLC \| Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Cambrian \| Mavrix
Divestitures
Percentage of ownership sold	51.00%	19.00%

Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Cash in excess of insurance limits	$ 50,117	$ 88,740

Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Restricted Cash
Short-term restricted cash	$ 4,469	$ 6,012
Canton Bonds
Restricted Cash
Short-term restricted cash	2,716	4,259
Standby letters of credit
Restricted Cash
Short-term restricted cash	$ 1,753	$ 1,753

Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Short-term investments
Amortized Cost	$ 128,641	$ 122,605
Gross Unrealized Gains/(Losses)	(45)	(59)
Estimated Fair Value	128,596	122,546
Municipal bonds and notes
Short-term investments
Amortized Cost	38,825	38,668
Gross Unrealized Gains/(Losses)	(24)	(16)
Estimated Fair Value	38,801	38,652
Zero coupon bonds
Short-term investments
Amortized Cost	5,013	3,308
Gross Unrealized Gains/(Losses)	3	(2)
Estimated Fair Value	5,016	3,306
Corporate bonds
Short-term investments
Amortized Cost	38,850	45,274
Gross Unrealized Gains/(Losses)	(24)	(41)
Estimated Fair Value	38,826	45,233
Certificates of deposit
Short-term investments
Amortized Cost	45,953	35,355
Estimated Fair Value	$ 45,953	$ 35,355

Other Receivables (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Other Receivables
Other receivables	$ 18,656	$ 56,223
Loans to customers to finance vehicle purchases
Other Receivables
Other receivables	8,982	8,257
Accrued customer billings
Other Receivables
Other receivables	5,730	10,143
Fuel tax and carbon credits
Other Receivables
Other receivables	155	34,250
Other
Other Receivables
Other receivables	$ 3,789	$ 3,573

Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Inventories
Raw materials and spare parts	$ 28,080	$ 31,389
Work in process	1,879	3,292
Finished goods	1,388	15
Total	$ 31,347	$ 34,696

Land, Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Land, Property and Equipment
Land, property and equipment, gross	$ 713,511		$ 713,511		$ 686,539
Less: accumulated depreciation	(193,087)		(193,087)		(172,270)
Land, property and equipment, net	520,424		520,424		514,269
Capitalized software costs	21,558		21,558		21,004
Accumulated amortization on the capitalized software costs	12,287		12,287		10,740
Amortization expense related to the capitalized software costs	831	$ 843	1,547	$ 1,611
Property and equipment purchases included in accounts payable and accrued liabilities	9,720	$ 16,360	9,720	$ 16,360
Land
Land, Property and Equipment
Land, property and equipment, gross	2,858		2,858		2,858
LNG liquefaction plants
Land, Property and Equipment
Land, property and equipment, gross	94,636		94,636		94,636
RNG plants
Land, Property and Equipment
Land, property and equipment, gross	45,451		45,451		45,359
Station equipment
Land, Property and Equipment
Land, property and equipment, gross	293,851		293,851		265,086
Trailers
Land, Property and Equipment
Land, property and equipment, gross	50,355		50,355		40,067
Other equipment
Land, Property and Equipment
Land, property and equipment, gross	79,929		79,929		74,796
Construction in progress
Land, Property and Equipment
Land, property and equipment, gross	$ 146,431		$ 146,431		$ 163,737

Investments in Other Entities (Details) $ in Thousands	3 Months Ended	6 Months Ended
Investments in Other Entities (Details) $ in Thousands	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)
Equity method investments
Loss from equity method investment	$ 345	$ 549
MCEP
Equity method investments
Ownership interest (as a percent)	50.00%	50.00%
Loss from equity method investment	$ 345	$ 549
Investment balance	$ 4,961	$ 4,961

Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Accrued Liabilities
Salaries and wages	$ 8,206	$ 9,041
Accrued gas and equipment purchases	15,904	12,340
Accrued property and other taxes	7,199	5,178
Accrued professional fees	742	1,084
Accrued employee benefits	4,125	3,208
Accrued warranty liability	2,423	2,302	$ 3,315	$ 2,545
Accrued interest	3,721	3,748
Other	9,007	19,859
Total	$ 51,327	$ 56,760

Warranty Liability (Details) - USD ($) $ in Thousands	6 Months Ended
Warranty Liability (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Jun. 30, 2014
Changes in the warrant liability
Warranty liability at beginning of year	$ 2,302	$ 2,545
Costs accrued for new warranty contracts and changes in estimates for pre-existing warranties	1,220	2,455
Service obligations honored	(1,099)	(1,685)
Warranty liability at end of period	$ 2,423	$ 3,315

Long-Term Debt (Details) $ / shares in Units, $ in Thousands	Aug. 27, 2013 USD ($) item	Jun. 14, 2013 USD ($) item $ / shares	Jun. 14, 2013 USD ($) $ / shares	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 11, 2011 USD ($)
Long-term debt
Restricted cash				$ 4,469	$ 6,012
7.5% Notes
Long-term debt
Principal amount of each debt instrument to be issued						$ 50,000
Interest rate (as a percent)						7.50%
Restricted cash				$ 0
7.5% Notes \| Maximum
Long-term debt
Financing commitment received						$ 150,000
7.5% Notes \| Boone Pickens
Long-term debt
Aggregate principal amount	$ 65,000
7.5% Notes \| Boone Pickens \| Common stock
Long-term debt
Number of shares of common stock into which Notes are convertible \| item	4,113,924
7.5% Notes \| Buyers
Long-term debt
Additional amount of advances under the obligation assumed		$ 50,000
Amount of advance funded			$ 50,000
Conversion price of shares (in dollars per share) \| $ / shares		$ 15.80	$ 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 \| item		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
7.5% Notes \| Green Energy Investment Holdings, LLC
Long-term debt
Principal amount transferred	$ 5,000
Aggregate principal amount	$ 80,000
7.5% Notes \| Green Energy Investment Holdings, LLC \| Common stock
Long-term debt
Number of shares of common stock into which Notes are convertible \| item	5,063,291

Long-Term Debt (Details 2) - SLG Notes $ / shares in Units, $ in Thousands		1 Months Ended	2 Months Ended
	Aug. 24, 2011 USD ($) item $ / shares	Apr. 30, 2012 USD ($) shares	Feb. 28, 2013 USD ($) shares	Mar. 01, 2012 USD ($)
Long-term debt
Financing commitment received	$ 150,000
Interest rate (as a percent)	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 \| item	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
Common stock
Long-term debt
Amount of principal and accrued interest under debt conversion		$ 1,003	$ 4,030
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

Long-Term Debt (Details 3) - GE Credit Agreement $ in Thousands		3 Months Ended		6 Months Ended
	Nov. 07, 2012 USD ($) item	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Long-term debt
Number of wholly owned subsidiaries through which the entity entered into a financing arrangement \| item	2
Line of credit limit \| $	$ 200,000
Number of LNG production facilities being financed \| item	2
Commitment fee on the unutilized loan amounts (as a percent)	0.50%
Minimum
Long-term debt
Percentage of the budgeted costs of the Projects which the Company agreed to pay	25.00%
Maximum
Long-term debt
Project completion period after the funding of the initial loans	30 months
Amortization period following conversion into term loan	8 years
Interest expense
Long-term debt
Commitment fee \| $		$ 253	$ 253	$ 503	$ 503
LIBOR
Long-term debt
Percentage of margin added to reference rate to determine interest rate on debt	7.00%
Reference rate minimum (as a percent)	1.00%

Long-Term Debt (Details 4) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Sep. 30, 2013 USD ($) item $ / shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)
Long-term debt
Payment of certain debt issuance costs		$ 914
5.25% Notes
Long-term debt
Interest rate (as a percent)	5.25%
Payment of certain debt issuance costs	$ 7,805
Amount of advance funded	$ 242,195
Conversion rate of debt instrument (shares per USD)	0.0641026
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 \| item	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%
Amount of sinking fund			$ 0
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 \| 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%

Long-Term Debt (Details 5) - Canton Bonds - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 19, 2014
Long-term debt
Principal amount with recourse to the Company	$ 1,000
Canton Renewables
Long-term debt
Debt issuance amount		$ 12,400
Coupon interest rate (as a percent)		6.60%

Long-Term Debt (Details 6) - USD ($) $ in Thousands	6 Months Ended
Long-Term Debt (Details 6) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Long-term debt
Total debt and capital lease obligations	$ 575,746	$ 570,670
Less amounts due within one year	(5,977)	(4,846)
Total long-term debt and capital lease obligations	569,769	565,824
7.5% Notes
Long-term debt
Total debt and capital lease obligations	150,000	150,000
SLG Notes
Long-term debt
Total debt and capital lease obligations	145,000	145,000
5.25% Notes
Long-term debt
Total debt and capital lease obligations	250,000	250,000
Canton Bonds
Long-term debt
Total debt and capital lease obligations	11,600	12,150
Principal amount of non-recourse debt	11,400	11,400
Capital lease obligations
Long-term debt
Total debt and capital lease obligations	$ 7,754	$ 2,692
Other debt
Long-term debt
Maximum interest rate on other debt borrowings	15.49%
Weighted average interest rate	6.67%	6.80%
Total debt and capital lease obligations	$ 11,392	$ 10,828

Net Loss Per Share (Details) - shares	1 Months Ended	3 Months Ended		6 Months Ended		10 Months Ended	12 Months Ended
Net Loss Per Share (Details) - shares	Dec. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2012
Basic and diluted:
Weighted-average number of common shares outstanding		91,480,998	94,859,587	91,399,478	94,768,462
Common stock \| GE Credit Agreement \| GE Warrant
Number of shares excluded from weighted average share calculation						4,000,000
Number of shares issuable upon exercise of warrants		5,000,000		5,000,000		5,000,000
Number of shares included in weighted average share calculation						1,000,000
Number of shares that became exercisable in the period	500,000						500,000

Net Loss Per Share (Details 2) - shares	6 Months Ended
Net Loss Per Share (Details 2) - shares	Jun. 30, 2015	Jun. 30, 2014
Stock Options
Net Loss Per Share
Anti-dilutive securities (in shares)	10,782,668	11,858,681
Series I Warrant Liabilities
Net Loss Per Share
Anti-dilutive securities (in shares)	6,130,682	2,130,682
Convertible Notes
Net Loss Per Share
Anti-dilutive securities (in shares)	35,185,979	35,185,979
Restricted Stock Units
Net Loss Per Share
Anti-dilutive securities (in shares)	2,972,252	2,080,336

Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2013
Stock-based compensation
Stock-based compensation expense	$ 2,663	$ 2,978	$ 5,353	$ 6,398
Stock-based compensation expense, net of tax	$ 2,663	$ 2,978	5,353	6,398
Stock Options
Stock-based compensation
Stock-based compensation expense			$ 2,843	3,819
Number of Shares
Outstanding at the beginning of the period			11,486,301
Options granted			465,000
Options exercised			(607,500)
Options forfeited			(561,133)
Outstanding at the end of the period	10,782,668		10,782,668
Exercisable at the end of the period	9,340,463		9,340,463
Weighted Average Exercise Price
Outstanding at the beginning of the period			$ 11.91
Options granted			6.15
Options exercised			2.96
Options forfeited			14.13
Outstanding at the end of the period	$ 12.05		12.05
Exercisable at the end of the period	$ 12.48		$ 12.48
Additional option disclosures
Options outstanding at the end of the period, weighted average remaining contractual term			5 years 4 days
Options exercisable at the end of the period, weighted average remaining contractual term			4 years 5 months 5 days
Other disclosures
Total unrecognized compensation cost related to non-vested shares	$ 4,979		$ 4,979
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized			1 year 7 months 6 days
Total fair value of shares vested			$ 4,314
Intrinsic value of all options exercised			$ 2,196	$ 1,419
Weighted-average assumption used for grants
Dividend yield			0.00%
Expected volatility, minimum			59.20%
Expected volatility, maximum			61.50%
Risk-free interest rate, minimum			1.70%
Risk-free interest rate, maximum			1.80%
Expected life			6 years
Stock-based compensation
Weighted-average grant date fair values of options granted			$ 3.42	$ 5.99
Clean Energy Renewables Plan \| Stock Options \| CERF
Stock-based compensation
Stock-based compensation expense			$ 538	$ 614
Number of shares reserved for issuance					150,000
Number of Shares
Outstanding at the beginning of the period			115,000
Options forfeited			(7,000)
Outstanding at the end of the period	108,000		108,000
Weighted Average Exercise Price
Outstanding at the beginning of the period			$ 40.80
Options forfeited			40.80
Outstanding at the end of the period	$ 40.80		$ 40.80
Additional option disclosures
Options outstanding at the end of the period, weighted average remaining contractual term			8 years 2 months 19 days
Other disclosures
Total unrecognized compensation cost related to non-vested shares	$ 1,384		$ 1,384
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized			10 months 24 days
NG Advantage Plan \| Stock Options \| NG Advantage
Stock-based compensation
Stock-based compensation expense			$ 12

Stock-Based Compensation (Details 2) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Stock-based compensation
Stock-based compensation expense	$ 2,663	$ 2,978	$ 5,353	$ 6,398
Market-Based RSUs
Stock-based compensation
Number of shares of common stock to be received for each performance unit			1
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%
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 Non-vested Shares
Outstanding at the beginning of the period			1,769,000
Outstanding at the end of the period	1,769,000		1,769,000
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period			$ 10.67
Outstanding at the end of the period	$ 10.67		$ 10.67
Weighted Average Remaining Contractual Term			1 year 1 month 6 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 1,180		$ 1,180
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized			7 months 6 days
Stock-based compensation expense			$ 885	1,775
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
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 \| Key Employee
Stock-based compensation
Vesting period			3 years
Number of Non-vested Shares
Outstanding at the beginning of the period			822,752
RSUs granted			419,500
RSUs forfeited			(39,000)
Outstanding at the end of the period	1,203,252		1,203,252
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period			$ 5.82
RSUs granted			6.05
RSUs forfeited			5.58
Outstanding at the end of the period	$ 5.91		$ 5.91
Weighted Average Remaining Contractual Term			2 years 6 months
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 5,643		$ 5,643
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized			2 years 4 months 24 days
Stock-based compensation expense			$ 1,050	$ 153
Service-Based RSUs \| Key Employee \| Vesting over the first year
Stock-based compensation
Vesting percentage			34.00%
Service-Based RSUs \| Key Employee \| Vesting over the second year
Stock-based compensation
Vesting percentage			33.00%
Service-Based RSUs \| Key Employee \| Vesting over the third year
Stock-based compensation
Vesting percentage			33.00%

Stock-Based Compensation (Details 3) $ in Thousands	3 Months Ended		6 Months Ended
Stock-Based Compensation (Details 3) $ in Thousands	Jun. 30, 2015 USD ($) shares	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) item shares	Jun. 30, 2014 USD ($)
Stock-based compensation
Stock-based compensation expense \| $	$ 2,663	$ 2,978	$ 5,353	$ 6,398
Employee Stock Purchase Plan
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 \| item			2
Number of shares reserved \| shares	2,500,000		2,500,000
Stock-based compensation expense \| $			$ 25	$ 37
Shares sold pursuant to the ESPP \| shares			105,257

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Taxes
Income tax expense	$ 740	$ 147	$ 1,594	$ 1,109

Fair Value Measurements (Details) - Fair value measured on recurring basis - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair Value Measurements
Estimated remaining life of Series I warrants	9 months 18 days
Minimum
Fair Value Measurements
Term of traded options	3 months
Maximum
Fair Value Measurements
Term of traded options	9 months
Certificates of deposit
Assets:
Available-for-sale securities	$ 45,953	$ 35,355
Municipal bonds and notes
Assets:
Available-for-sale securities	38,801	38,652
Zero coupon bonds
Assets:
Available-for-sale securities	5,016	3,306
Corporate bonds
Assets:
Available-for-sale securities	38,826	45,233
Series I warrants
Liabilities:
Derivative liability	833	1,416
Level 2 \| Certificates of deposit
Assets:
Available-for-sale securities	45,953	35,355
Level 2 \| Municipal bonds and notes
Assets:
Available-for-sale securities	38,801	38,652
Level 2 \| Zero coupon bonds
Assets:
Available-for-sale securities	5,016	3,306
Level 2 \| Corporate bonds
Assets:
Available-for-sale securities	38,826	45,233
Level 3 \| Series I warrants
Liabilities:
Derivative liability	$ 833	$ 1,416

Fair Value Measurements (Details 2) - Series I Warrant Liabilities - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Reconciliation of the beginning and ending balances of items measured at fair value using significant unobservable inputs (Level 3)
Beginning Balance	$ 1,416	$ 7,164
Total gain included in earnings	(583)	(2,169)
Ending Balance	$ 833	$ 4,995

Fair Value Measurements (Details 3) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Nonrecurring
Unobservable Input
Impairments of long-lived assets	$ 0	$ 0
Level 3 \| Series I Warrant Liabilities \| Black-Scholes Model
Unobservable Input
Current market price of the Company's common stock (in dollars per share)	$ 5.62
Exercise price of the warrant (in dollars per share)	$ 12.68
Dividend yield (as a percent)	0.00%
Remaining term of the warrant	9 months 29 days
Implied volatility of the Company's common stock (as a percent)	80.36%
Level 3 \| Series I Warrant Liabilities \| Black-Scholes Model \| Simple average
Unobservable Input
Assumed discount rate (as a percent)	0.28%

Recently Adopted Accounting Changes and Recently Issued Accounting Standards (Details) $ in Thousands	Jun. 30, 2015 USD ($)
Recently Adopted Accounting Changes and Recently Issued Accounting Standards
Debt issuance costs	$ 4,840

Alternative Fuels Excise Tax Credit (Details) $ in Thousands	12 Months Ended	99 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2014 $ / gal
Alternative Fuels Excise Tax Credit
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent)		0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon)		0.50
VETC credits recognized as revenue \| $	$ 28,359