Clean Energy Fuels (CLNE) 10-K 13 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 06, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	Clean Energy Fuels Corp.
Entity Central Index Key	1,368,265
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding (in shares)		152,394,550
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 343,111,833

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 36,081	$ 36,119
Restricted cash	1,127	6,996
Short-term investments	141,462	73,718
Accounts receivable, net of allowance for doubtful accounts of $1,063 and $3,676 as of December 31, 2016 and 2017, respectively	63,961	79,432
Other receivables	19,235	21,934
Inventory	35,238	29,544
Prepaid expenses and other current assets	7,793	14,021
Total current assets	304,897	261,764
Land, property and equipment, net	367,305	483,923
Notes receivable and other long-term assets, net	21,397	16,377
Investments in other entities	30,395	3,475
Goodwill	64,328	93,018
Intangible assets, net	3,590	38,700
Total assets	791,912	897,257
Current liabilities:
Current portion of debt and capital lease obligations	139,699	5,943
Accounts payable	17,901	23,637
Accrued liabilities	42,268	52,601
Deferred revenue	3,432	7,041
Total current liabilities	203,300	89,222
Long-term portion of debt and capital lease obligations	120,388	241,433
Long-term debt, related party	0	65,000
Other long-term liabilities	18,566	7,915
Total liabilities	342,254	403,570
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.0001 par value. Authorized 1,000,000 shares; issued and outstanding no shares	0	0
Common stock, $0.0001 par value. Authorized 224,000,000 shares; issued and outstanding 145,538,063 shares and 151,650,969 shares as of December 31, 2016 and 2017, respectively	15	15
Additional paid-in capital	1,111,432	1,090,361
Accumulated deficit	(683,570)	(603,836)
Accumulated other comprehensive loss	(887)	(17,675)
Total Clean Energy Fuels Corp. stockholders' equity	426,990	468,865
Noncontrolling interest in subsidiary	22,668	24,822
Total stockholders' equity	449,658	493,687
Total liabilities and stockholders' equity	$ 791,912	$ 897,257

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,063	$ 3,676
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	1,000,000	1,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	224,000,000	224,000,000
Common stock, issued (in shares)	145,538,063	151,650,969
Common stock, outstanding (in shares)	145,538,063	151,650,969

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Product revenue	$ 287,292	$ 351,038	$ 329,168
Service revenue	54,307	51,618	55,152
Total revenue	341,599	402,656	384,320
Cost of sales (exclusive of depreciation and amortization shown separately below):
Product cost of sales	216,413	229,958	230,621
Service cost of sales	26,258	25,592	27,864
Inventory valuation provision	13,158	0	0
Selling, general and administrative	95,669	105,481	112,239
Depreciation and amortization	56,614	59,262	55,219
Asset impairments and other charges	67,934	0	0
Total operating expenses	476,046	420,293	425,943
Operating loss	(134,447)	(17,637)	(41,623)
Interest expense	(17,751)	(29,595)	(95,813)
Interest income	1,497	827	843
Other income (expense), net	139	(306)	2,627
Loss from equity method investments	(131)	(22)	(815)
Gain from extinguishment of debt, net	3,195	34,348	0
Gain from sale of certain assets of subsidiary	70,658	0	937
Loss from formation of equity method investment	(6,465)	0	0
Loss before income taxes	(83,305)	(12,385)	(133,844)
Income tax benefit (expense)	1,914	(1,339)	(1,614)
Net loss	(81,391)	(13,724)	(135,458)
Loss attributable to noncontrolling interest	2,154	1,571	1,216
Net loss attributable to Clean Energy Fuels Corp.	$ (79,237)	$ (12,153)	$ (134,242)
Loss per share:
Basic and diluted (in dollars per share)	$ (0.53)	$ (0.10)	$ (1.47)
Weighted-average common shares outstanding:
Basic and diluted (in shares)	150,430,239	119,395,423	91,607,578

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net loss	$ (81,391)	$ (13,724)	$ (135,458)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2015, 2016 and 2017	(113)	1,567	(9,653)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2015, 2016 and 2017	0	1,652	(8,078)
Unrealized gains on available-for-sale securities, net of $0 tax in 2015, 2016 and 2017	189	79	6
Release of foreign currency translation adjustments on contribution of subsidiary into equity method investment	16,712	0	0
Total other comprehensive income (loss)	16,788	3,298	(17,725)
Comprehensive loss	(64,603)	(10,426)	(153,183)
Clean Energy Fuels Corp
Net loss	(79,237)	(12,153)	(134,242)
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments net of $0 tax in 2015, 2016 and 2017	(113)	1,567	(9,653)
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2015, 2016 and 2017	0	1,652	(8,078)
Unrealized gains on available-for-sale securities, net of $0 tax in 2015, 2016 and 2017	189	79	6
Release of foreign currency translation adjustments on contribution of subsidiary into equity method investment	16,712	0	0
Total other comprehensive income (loss)	16,788	3,298	(17,725)
Comprehensive loss	(62,449)	(8,855)	(151,967)
Noncontrolling Interest in Subsidiary
Net loss	(2,154)	(1,571)	(1,216)
Other comprehensive income (loss), net of tax:
Comprehensive loss	$ (2,154)	$ (1,571)	$ (1,216)

CONSOLIDATED STATEMENTS OF COM6

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (PARENTHETICAL) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Foreign currency translation adjustment, tax	$ 0	$ 0	$ 0
Foreign currency adjustments on intra-entity long-term investments, tax	0	0	0
Unrealized gains on available-for sale securities, tax	0	0	0
Unrecognized gains on derivatives, tax	$ 0	$ 0	$ 0

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Subsidiary
Beginning balance (in shares) at Dec. 31, 2014		90,203,344
Beginning balance at Dec. 31, 2014	$ 465,035	$ 9	$ 898,106	$ (457,441)	$ (3,248)	$ 27,609
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of offering costs (in shares)		2,179,373
Issuance of common stock, net of offering costs	6,314	$ 0	6,314
Stock-based compensation	10,779		10,779
Net loss	(135,458)			(134,242)		(1,216)
Accumulated other comprehensive income (loss)	(17,725)				(17,725)
Ending balance (in shares) at Dec. 31, 2015		92,382,717
Ending balance at Dec. 31, 2015	328,945	$ 9	915,199	(591,683)	(20,973)	26,393
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of offering costs (in shares)		32,889,517
Issuance of common stock, net of offering costs	101,120	$ 4	101,116
Stock-based compensation	8,092		8,092
Net loss	(13,724)			(12,153)		(1,571)
Issuance of common stock in connection with debt extinguishment (in shares)		20,265,829
Issuance of common stock in connection with debt extinguishment	65,956	$ 2	65,954
Accumulated other comprehensive income (loss)	$ 3,298				3,298
Ending balance (in shares) at Dec. 31, 2016	151,650,969	145,538,063
Ending balance at Dec. 31, 2016	$ 493,687	$ 15	1,090,361	(603,836)	(17,675)	24,822
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of offering costs (in shares)		6,112,906
Issuance of common stock, net of offering costs	12,454		12,454
Stock-based compensation	8,423		8,423
Net loss	(81,391)			(79,237)		(2,154)
Accumulated other comprehensive income (loss)	$ 16,788				16,788
Ending balance (in shares) at Dec. 31, 2017	145,538,063	151,650,969
Ending balance at Dec. 31, 2017	$ 449,658	$ 15	$ 1,111,432	$ (683,570)	$ (887)	$ 22,668

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (81,391)	$ (13,724)	$ (135,458)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	56,614	59,262	55,219
Provision for doubtful accounts, notes and inventory	19,835	4,374	2,872
Stock-based compensation expense	8,123	8,092	10,779
Amortization of debt issuance cost	847	1,527	2,969
Non-cash interest charge related to a terminated credit agreement	0	0	54,925
Accretion of notes payable	0	0	57
Gain on extinguishment of debt, net	(3,195)	(34,348)	0
Gain from sale of certain assets of subsidiary	(70,658)	0	(937)
Loss from formation of equity method investment	6,465	0	0
Asset impairments and other charges	58,061	0	0
Changes in operating assets and liabilities, net of assets and liabilities acquired and disposed:
Accounts and other receivables	6,881	30,171	3,426
Inventory	963	(1,520)	5,191
Prepaid expenses and other assets	5,643	347	2,876
Accounts payable	(8,964)	(764)	(12,005)
Deferred revenue	9,268	(3,479)	(4,134)
Accrued expenses and other	(13,919)	(3,876)	2,124
Net cash provided by (used in) operating activities	(5,427)	46,062	(12,096)
Cash flows from investing activities:
Purchases of short-term investments	(340,194)	(137,023)	(158,840)
Maturities and sales of short-term investments	272,220	165,695	176,969
Purchases and deposits on property and equipment	(36,307)	(23,640)	(51,415)
Loans made to customers	(894)	(2,816)	(4,279)
Payments on and proceeds from sales of loans receivable	1,102	842	928
Restricted cash	1,578	(2,634)	1,650
Cash received from sale certain assets of subsidiary, net of cash transferred	154,489	0	1,118
Cash contributed in formation of equity method investment	(2,404)	0	0
Investments in other entities	(1,928)	(833)	0
Capital from equity method investment	0	3,031	0
Acquisitions, net of cash acquired	0	(1,550)	0
Net cash provided by (used in) investing activities	47,662	1,072	(33,869)
Cash flows from financing activities:
Issuances of common stock	10,767	103,591	7,197
Fees paid for issuances of common stock	(638)	(2,283)	(883)
Proceeds from debt instruments	9,765	7,412	384
Proceeds from revolving line of credit	312	73,508	31
Repayment of borrowing under revolving line of credit	(23,812)	(50,027)	(64)
Repayment of capital lease obligations and debt instruments	(30,707)	(187,824)	(6,258)
Payment to holders of stock options in subsidiaries	(8,850)	0	0
Net cash provided by (used in) financing activities	(43,163)	(55,623)	407
Effect of exchange rates on cash and cash equivalents	890	884	(3,099)
Net decrease in cash and cash equivalents	(38)	(7,605)	(48,657)
Cash and cash equivalents, beginning of year	36,119	43,724	92,381
Cash and cash equivalents, end of year	36,081	36,119	43,724
Supplemental disclosure of cash flow information:
Income taxes paid	344	1,012	890
Interest paid, net of $835, $447 and $103 capitalized, respectively	$ 17,048	$ 29,774	$ 37,662

CONSOLIDATED STATEMENTS OF CAS9

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Cash Flows [Abstract]
Interest paid, capitalized	$ 17,048	$ 29,774	$ 37,662

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company Clean Energy Fuels Corp., together with its majority and wholly owned subsidiaries (hereinafter collectively referred to as the "Company," unless the context or the use of the term indicates or requires otherwise) is engaged in the business of selling natural gas as an alternative fuel for vehicle fleets and related natural gas fueling solutions to its customers, primarily in the United States and Canada. The Company's principal business is supplying renewable natural gas ("RNG"), compressed natural gas (“CNG”) and liquefied natural gas (“LNG”) (RNG can be delivered in the form of CNG or LNG) for light, medium and heavy-duty vehicles and providing operation and maintenance ("O&M") services for vehicle fleet customer stations. As a comprehensive solution provider, the Company also designs, builds, operates and maintains fueling stations; sells and services natural gas fueling compressors and other equipment used in CNG stations and LNG stations; offers assessment, design and modification solutions to provide operators with code-compliant service and maintenance facilities for natural gas vehicle fleets; transports and sells CNG and LNG via “virtual” natural gas pipelines and interconnects; procures and sells RNG; sells tradable credits it generates by selling RNG and conventional natural gas as a vehicle fuel, including Renewable Identification Numbers ("RIN Credits" or "RINs") under the federal Renewable Fuel Standard Phase 2 and credits under the California and the Oregon Low Carbon Fuel Standards (collectively, "LCFS Credits"); helps its customers acquire and finance natural gas vehicles; and obtains federal, state and local credits, grants and incentives. In addition, for all periods presented before March 31, 2017, the Company produced RNG at its own production facilities, and for all periods presented before December 29, 2017, the Company manufactured, sold and serviced natural gas fueling compressors and other equipment used in CNG stations. See Note 3 for more information. Basis of Presentation The accompanying consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments necessary to state fairly the Company's financial position, results of operations, comprehensive loss and cash flows in accordance with accounting principles generally accepted in the United States of America ("US GAAP"). All intercompany accounts and transactions have been eliminated in consolidation. Reclassifications Prior period Gain from change in fair value of derivative warrants of $1,414 and $22 for the years ended December 31, 2015 and 2016, respectively, were reclassified from a separate line item into Selling, general and administrative in the consolidated statements of operations, and the same amounts were reclassified into Accrued expenses and other in the consolidated statements of cash flows to conform to the classifications used to prepare the consolidated financial statements for the year ended December 31, 2017 . In addition, Deferred revenue of $4,134 and $3,479 for the years ended December 31, 2015 and 2016, respectively were reclassified from Accrued expenses and other as a separate line item in the consolidated statements of cash flows to conform to current period presentation. These reclassifications had no material impact on the Company’s financial position, results of operations or cash flows as previously reported. Prior period Interest income of $843 for the year ended December 31, 2015 was reclassified from Interest expense, net to a separate line item, to conform to the classifications used to prepare the consolidated financial statements for the years ended December 31, 2016 and 2017 . This reclassification had no material impact on the Company’s financial position, results of operations or cash flows as previously reported. Use of Estimates The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and may result in material effects on the Company's operating results and financial position. Significant estimates made in preparing the consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, fair value measurements and stock-based compensation expense. Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of three months or less on the date of acquisition to be cash equivalents. Fair Value of Financial Instruments The carrying values of the Company's financial instruments, including cash and cash equivalents, restricted cash, short-term investments, accounts receivables, other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable, approximate their respective fair values. Inventory Inventory consists of raw materials and spare parts, work in process and finished goods and is stated at the lower of cost (first-in, first-out) or net realizable value. 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, 2016 and 2017 : 2016 2017 Raw materials and spare parts (1) $ 24,843 $ 35,145 Work in process 845 — Finished goods 3,856 93 Total inventory $ 29,544 $ 35,238 (1) During the year ended December 31, 2017, $19,394 in station parts were reclassified from construction in progress within Land, property, and equipment, net, into Inventory in the consolidated balance sheets because they will primarily be used for stations to be sold. See Note 2 for more information. Property and Equipment Property and equipment are recorded at cost. Depreciation and amortization are recognized over the estimated useful lives of the assets using the straight-line method. The estimated useful lives of depreciable assets are three to twenty years for LNG liquefaction plant assets, up to 10 years for station equipment and LNG trailers, and three to seven years for all other depreciable assets. Leasehold improvements are amortized over the shorter of their estimated useful lives or related lease terms. Periodically, the Company receives grant funding to assist in the financing of natural gas fueling station construction. The Company records the grant proceeds as a reduction of the cost of the respective asset. Total grant proceeds received were approximately $4,292 , $3,295 , and $4,360 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Long-Lived Assets The Company reviews the carrying value of its long-lived assets, including property and equipment and intangible assets with finite useful lives, for impairment whenever events or changes in circumstances indicate that the carrying value of an asset or asset group may not be recoverable. Events that could result in an impairment review include, among others, a significant decrease in the operating performance of a long-lived asset or asset group or the decision to close a fueling station. Impairment testing involves a comparison of the sum of the undiscounted future cash flows of the asset or asset group to its carrying amount. If the sum of the undiscounted future cash flows exceeds the carrying amount, then no impairment exists. If the carrying amount exceeds the sum of the undiscounted future cash flows, then a second step is performed to determine the amount of impairment, if any, to be recognized. An impairment loss is recognized to the extent that the carrying amount of the asset or asset group exceeds its fair value. The fair value of the asset or asset group is based on estimated discounted future cash flows of the asset or asset group using a discount rate commensurate with the related risk. The estimate of future cash flows requires management to make assumptions and to apply judgment, including forecasting future sales and expenses and estimating useful lives of the assets. These estimates can be affected by a number of factors, including, among others, future results, demand, and economic conditions, many of which can be difficult to predict. In the third quarter of the year ended December 31, 2017, the Company recorded asset impairment charges of $32,274 related to one of its subsidiaries, IMW Industries Ltd. ("IMW") (formerly known as Clean Energy Compression Corp.) ("CEC") and $20,384 related to certain station closures (see Note 2 for more information). There were no impairments of the Company’s long-lived assets in the years ended December 31, 2015 or 2016 . Intangible assets with finite useful lives are amortized over their respective estimated useful lives using the straight-line method. The estimated useful lives of intangible assets with finite useful lives are from two to 20 years for technology, one to eight years for customer relationships, one to 10 years for acquired contracts, two to 10 years for trademarks and trade names, and three years for non-compete agreements. The Company's intangible assets as of December 31, 2016 and 2017 were as follows: 2016 2017 Technology $ 54,400 $ — Customer relationships 16,576 5,376 Acquired contracts 4,384 4,384 Trademark and trade names 8,200 2,700 Non-compete agreements 2,060 860 Total intangible assets 85,620 13,320 Less accumulated amortization (37,591 ) (9,730 ) Foreign currency rate change (9,329 ) — Net intangible assets $ 38,700 $ 3,590 Amortization expense for intangible assets was $5,539 , $5,794 , and $5,065 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Estimated amortization expense for the five years and thereafter succeeding the year ended December 31, 2017 is approximately $1,393 , $973 , $765 , $459 , $0 and $0 , respectively. Goodwill Goodwill represents the excess of costs incurred over the fair value of the net assets of acquired businesses. The Company assesses its goodwill using either a qualitative or quantitative approach to determine whether it is more likely than not that the fair value of its reporting unit is less than its carrying value. The Company is required to use judgment when applying the goodwill impairment test, including, among other considerations, the identification of reporting unit(s), the assessment of qualitative factors, and the estimation of fair value of a reporting unit in the quantitative approach. The Company determined that it is a single reporting unit for the purpose of goodwill impairment tests. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review. The qualitative goodwill assessment includes the potential impact on a reporting unit's fair value of certain events and circumstances, including its enterprise value, macroeconomic conditions, industry and market considerations, cost factors, and other relevant entity-specific events. If it is determined, based upon the qualitative assessment, that it is more likely than not that the reporting unit's fair value is less than its carrying amount, then a quantitative impairment test is performed. The quantitative assessment estimates the reporting unit's fair value based on its enterprise value plus an assumed control premium as evidence of fair value. The estimates used to determine the fair value of the reporting unit may change based on results of operations, macroeconomic conditions, stock price fluctuations, or other factors. Changes in these estimates could materially affect our assessment of the fair value and goodwill impairment for the reporting unit. During the years ended December 31, 2015 and 2016 , the Company utilized the qualitative approach and concluded there were no indicators of impairment to goodwill. During the third quarter of the year ended December 31, 2017 , as a result of the asset impairment charges recorded for intangible assets and stations (described previously and in Note 2 ), the Company determined that sufficient indicators of potential impairment existed to require an interim goodwill test of its one reporting unit prior to the annual test performed in the fourth quarter of 2017 . The goodwill test was performed by computing the fair value of the reporting unit and comparing it to the carrying value using a quantitative assessment. Based on the results of the goodwill test, the Company concluded that it is more likely than not that the fair value of its reporting unit exceeds its carrying amount and thus no impairment existed. The annual impairment test was subsequently performed on October 1 using the quantitative assessment and the Company concluded no impairment existed. The qualitative assessment was also performed as of December 31, 2017 as a result of volatility in the Company's market capitalization subsequent to October 1, with no changes in the conclusions during the annual impairment test. The following table summarizes the activity related to the carrying amount of goodwill: Balance as of December 31, 2015 $ 91,967 Foreign currency translation adjustment 1,051 Balance as of December 31, 2016 93,018 Goodwill reduced during the year (1) (30,154 ) Foreign currency translation adjustment 1,464 Balance as of December 31, 2017 $ 64,328 (1) The Company reduced its goodwill balance by $26,576 when it sold certain assets of its subsidiary, Clean Energy Renewable Fuels ("Renewables"), on March 31, 2017, and by $3,578 when it contributed CEC to SAFE&CEC S.r.l. on December 29, 2017 (all described in Note 3 ). Revenue Recognition The Company recognizes revenue from various sources. The table below and the following discussion describe the Company’s revenue by group of similar products, services or other revenue sources. Year Ended December 31, (in thousands) 2015 2016 2017 Volume -Related $ 260,629 $ 283,814 $ 264,880 Compressor Sales 54,497 27,262 23,527 Station Construction Sales 37,830 64,942 51,854 AFTC (1) 30,986 26,638 — Other 378 — 1,338 $ 384,320 $ 402,656 $ 341,599 (1) Formerly known as VETC. Volume -Related The Company’s volume related revenue primarily consists of CNG, LNG and RNG fuel sales, RINs and LCFS Credits sales and O&M services. This revenue is recognized when the following four criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred and title and the risks and rewards of ownership have been transferred to the customer or services have been rendered; (iii) the price is fixed or determinable; and (iv) collectability is reasonably assured. Applying these factors, the Company typically recognizes revenue from the sale of natural gas fuel at the time the fuel is dispensed or, in the case of LNG sales agreements, delivered to the customers' storage facilities. The Company recognizes revenue from O&M service agreements as the related services are provided. The Company generates RIN Credits when it sells RNG as a vehicle fuel in the United States, and it generates LCFS Credits when it sells RNG and conventional natural gas for use as a vehicle fuel in California and Oregon. The Company can sell these credits to third parties who need the credits to comply with federal and state requirements. RIN and LCFS Credits are included in volume related revenues. The Company recognizes revenue from the generation of these credits when it has an agreement in place to sell the credits at a fixed or determinable price. Compressor Sales Before completion of the CEC Combination (see Note 3 ), the Company recognized compression revenue through its former subsidiary CEC when it sold non-lubricated natural gas fueling compressors and other equipment. CEC used the percentage-of-completion method of accounting to recognize revenue because its projects were small and it was able to demonstrate that it could reasonably estimate costs to complete. In these circumstances, revenue was recognized based on costs incurred in relation to total estimated costs to be incurred for a project. Station Construction Sales Beginning January 1, 2016, the Company began using the percentage of completion method to recognize revenue for station construction projects using the cost-to-cost method. Under this method, the Company estimates the percentage of completion of a project based on the costs incurred to date for the associated contract in comparison to the estimated total costs for such contract at completion. Historically, the Company recognized revenue on station construction projects using the completed contract method because the Company did not have a reliable means to make estimates of the percentage of the contract completed. Under the completed contract method, the construction projects were considered substantially complete at the earlier of customer acceptance of the fueling station or the time when fuel dispensing activities at the station began. The sale of compressors and related equipment continues to be recognized under the percentage of completion method as in previous periods. Effective January 1, 2016, the Company implemented a cost tracking system that provides for a detailed tracking of costs incurred on its station construction projects on a project by project basis. The Company also changed related accounting activities and processes to timely identify and monitor costs. As a result of this implementation, the Company is able to make reliable estimates as to the percentage of a project that is complete at the end of each reporting period. Station construction contracts are generally short-term, except for certain larger and more complex stations, which can take up to 24 months to complete. Management evaluates the performance of contracts on an individual contract basis. Contract price and cost estimates are reviewed periodically as work progresses and adjustments proportionate to the percentage of completion are reflected in contract revenues in the reporting period when such estimates are revised. The nature of accounting for contracts is such that refinements of estimates to account for changing conditions and new developments are continuous and characteristic of the process. Many factors that can affect contract profitability may change during the performance period of a contract, including differing site conditions, the availability of skilled contract labor, the performance of major suppliers and subcontractors, and unexpected changes in material costs. Changes to these factors may result in revisions to costs and income, which are recognized in the period in which the revisions become known. Provisions for estimated losses on uncompleted contracts are made in the period in which such losses become known. During the year ended December 31, 2017 , there were no significant losses on open contracts. The Company considers unapproved change orders to be contract variations for which the customer has approved the change of scope but an agreement has not been reached as to an associated price change. Change orders that are unapproved as to both price and scope are evaluated as claims. Claims have historically been insignificant. There were no significant unapproved change orders, claims, contract penalties, settlements or changes in contract estimates during the year ended December 31, 2017 . In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction of and sale of a station, providing O&M services to the station, and sale of fuel to the customer. The Company evaluates the separability of revenues based on Financial Accounting Standards Board ("FASB") authoritative guidance, which provides a framework for establishing whether or not a particular arrangement with a customer has one or more revenue elements, and allows the Company to use a combination of internal and external objective and reliable evidence to develop management's best estimate of the fair value of the contract elements. If the arrangement contains a lease, the Company uses the existing evidence of fair value to separate the lease from the other elements in the arrangement. The arrangement's consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the estimated relative selling price of each deliverable, which is determined based on the historical data derived from the Company's stand-alone projects. The revenue allocated to the construction of the station is recognized using the percentage of completion method. The revenue allocated to the O&M services is recognized ratably over the term of the arrangement and sale of fuel is recognized as the fuel is delivered. See the discussion under “Alternative Fuels Excise Tax Credit” below for more information. Other The Company collects and remits taxes assessed by various governmental authorities that are imposed on and concurrent with revenue-producing transactions between the Company and our customers. These taxes may include, but are not limited to, fuel, sales and value-added taxes. The Company reports the collection of these taxes on a net basis. Alternative Fuels Excise Tax Credit Under separate pieces of U.S. federal legislation, the Company has been eligible to receive a federal alternative fuels tax credit (“AFTC,” formerly known as VETC) for its natural gas vehicle fuel sales made between October 1, 2006 and December 31, 2017. The AFTC, which had previously expired on December 31, 2016, was reinstated on February 9, 2018 to apply to vehicle fuel sales made from January 1, 2017 through December 31, 2017. The AFTC credit is equal to $0.50 per gasoline gallon equivalent of CNG that the Company sold as vehicle fuel, $0.50 per liquid gallon of LNG that the Company sold as vehicle fuel through 2015, and $0.50 per diesel gallon of LNG that the Company sold as vehicle fuel in 2016 and 2017. Based on the service relationship with its customers, either the Company or its customers claims the credit. The Company records its AFTC credits, if any, as revenue in its consolidated statements of operations because the credits are fully payable and do not need to offset income tax liabilities to be received. As such, the credits are not deemed income tax credits under the accounting guidance applicable to income taxes. As a result of the legislation being signed into law in 2018, all AFTC revenue for vehicle fuel the Company sold in the 2017 calendar year, will be recognized and collected subsequent to December 31, 2017. AFTC revenue recognized for years ended December 31, 2015 , 2016 and 2017 was $30,986 , $26,638 and $0 , respectively. LNG Transportation Costs The Company records the costs incurred to transport LNG to its customers in the line item Product cost of sales in the accompanying consolidated statements of operations. Advertising Costs Advertising costs are expensed as incurred. Advertising costs were $44 , $15 and $311 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Stock-Based Compensation The Company recognizes compensation expense for all stock‑based payment arrangements over the requisite service period of the award. For stock options, the Company determines the grant date fair value using the Black‑Scholes option pricing model, which requires the input of certain assumptions, including the expected life of the stock‑based payment awards, stock price volatility and risk‑free interest rates. For restricted stock units, the Company determines the grant date fair value based on the closing market price of its common stock on the date of grant. Income Taxes Income taxes are computed using the asset and liability method. Under this method, deferred income taxes are recognized by applying enacted statutory tax rates applicable to future years to differences between the tax bases and financial carrying amounts of existing assets and liabilities. The impact on deferred taxes of changes in tax rates and laws, if any, are applied to the years during which temporary differences are expected to be settled and are reflected in the consolidated financial statements in the period of enactment. Valuation allowances are established when management determines it is more likely than not that deferred tax assets will not be realized. When evaluating the need for a valuation analysis, we use estimates involving a high degree of judgment including projected future US GAAP income and the amounts and estimated timing of the reversal of any deferred tax assets and liabilities. The Company has a recognition threshold and a measurement attribute for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities based on the technical merits of the position. The amount recognized is measured as the largest amount of benefit that has a greater than 50 percent likelihood of being realized upon ultimate settlement. The Company recognizes potential accrued interest and penalties related to unrecognized tax benefit in income tax expense. The Company operates within multiple domestic and foreign taxing jurisdictions and is subject to audit in these jurisdictions. These audits can involve complex issues, which may require an extended period of time to resolve. Although the Company believes that adequate consideration has been given to these issues, it is possible that the ultimate resolution of these issues could be significantly different than originally estimated. Net Loss Per Share Basic net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period. Diluted net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period and potentially dilutive securities outstanding during the period, and therefore reflects the dilution from common shares that may be issued upon exercise or conversion of these potentially dilutive securities, such as stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock awards and warrants is computed under the treasury stock method. The dilutive effect of convertible notes and restricted stock units is computed under the if-converted method. Potentially dilutive securities are excluded from the computations of diluted net loss per share if their effect would be antidilutive. Although these securities were antidilutive for these periods, they could be dilutive in future periods. The following potentially dilutive securities have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive: 2015 2016 2017 Stock options 11,487,938 11,467,796 8,613,854 Warrants 2,130,682 — — Convertibles notes 35,185,979 16,573,799 14,991,521 Restricted stock units 3,419,776 2,072,304 1,832,575 Foreign Currency Translation The Company uses the local currency as the functional currency of its foreign subsidiaries. Accordingly, all assets and liabilities outside the United States are translated into U.S. dollars at the rate of exchange in effect at the balance sheet date. Revenue and expense items are translated at the weighted-average exchange rates prevailing during the period. Foreign currency translation adjustments are recorded as accumulated other comprehensive loss in stockholders' equity. Foreign currency transactions occur when there is a transaction denominated in other than the respective entity's functional currency. The Company records the changes in the exchange rate for these transactions in the consolidated statements of operations. For the years ended December 31, 2015 , 2016 and 2017 , foreign exchange transaction gains and (losses) were included in other income (expense) in the accompanying consolidated statements of operations and were $975 , $132 and $(246) , respectively. Comprehensive Loss Comprehensive loss is defined as the change in equity (net assets) of a business enterprise during the period from transactions and other events and circumstances from non-owner sources. The difference between net loss and comprehensive loss for the years ended December 31, 2015 , 2016 and 2017 was primarily comprised of the Company's foreign currency translation adjustments. Concentration of Credit Risk Credit is extended to all customers based on financial condition, and collateral is generally not required. Concentrations of credit risk with respect to trade receivables are limited because of the large number of customers comprising the Company's customer base and dispersion across many different industries and geographies. Certain international customers, however, have historically been slower to pay on trade receivables. Accordingly, the Company continually monitors collections and payments from its customers and maintains a provision for estimated credit losses based upon its historical experience and any specific customer collection issues that it has identified. Although credit losses have historically been within the Company's expectations and the provisions established, the Company cannot guarantee that it will continue to experience the same credit loss rates that it has in the past. The Company places its cash and cash equivalents with high credit quality financial institutions. At times, such investments may be in excess of the Federal Deposit Insurance Corporation ("FDIC"), and other foreign insurance limits. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash deposits. The amounts in excess of FDIC insurance limits were $34,439 and $34,709 as of December 31, 2016 and 2017 , respectively. Recently Adopted Accounting Changes and Recently Issued Accounting Standards Recently Adopted Accounting Changes In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope Modification Accounting . This ASU allows entities to make certain changes to awards without accounting for them as modifications. It does not change the accounting for modifications. Specifically, an entity will not apply modification accounting if all of the following are the same immediately before and after the change: (1) the award's fair value (or calculated value or intrinsic value if those measurement methods are used), (2) the award's vesting conditions, and (3) the award's classification as an equity or liability instrument. The new standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. Early adoption is permitted for interim or annual periods in which the financial statements have not been issued. The Company elected to early adopt this ASU during the year ended December 31, 2017, which did not have any impact on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new ASU, goodwill impairment testing will be performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value. The new standard is effective for annual and interim goodwill impairment tests in fiscal years beginning after December 15, 2019, which for the Company is the first quarter of 2020 and should be applied on a prospective basis. Early adoption is permitted for annual or interim goodwill impairment testing performed after January 1, 2017. The Company elected to early adopt this ASU during the year ended December 31, 2017, which did not have any impact on its consolidated financial

Asset Impairments, Other Charge

Asset Impairments, Other Charges, and Inventory Valuation Provision	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Asset Impairments, Other Charges, and Inventory Valuation Provision	Asset Impairments, Other Charges, and Inventory Valuation Provision In light of continuing low oil prices and the current state of natural gas vehicle adoption, among other factors, during the third quarter of the year ended December 31, 2017, the Company undertook an evaluation of its operations with the intent of minimizing and eliminating assets it believed were underperforming. As a result of this evaluation, the Company identified certain of its fueling stations where the current and projected natural gas volume and profitability levels are not expected to be sufficient to support the Company’s investment in the fueling station assets, and the Company decided to close these stations. The Company also reduced its workforce and took other steps to reduce overhead costs as a result of this evaluation, in an effort to lower its operating expenses going forward. In addition, this evaluation resulted in a strategic shift in how the Company viewed its natural gas compressor business, operated by CEC. In an effort to increase the scale and reach and improve the financial prospects of the Company’s investment in this business, the Company entered into an investment agreement with a strategic partner in November 2017, pursuant to which both parties combined their respective natural gas compressor businesses (see Note 3 for more information). As a result of these decisions and the steps taken to implement them, the Company incurred, during the year ended December 31, 2017 and on a pre-tax basis, aggregate cash and non-cash charges related to asset impairments and other charges, and a non-cash inventory valuation charge. In addition, the Company incurred a cash charge related to a payment for LCFS Credits during the fourth quarter of 2017. The following table summarizes these charges: Year Ended December 31, 2017 Workforce reduction and related charges $ 3,057 CEC asset impairments 32,274 Station closures and related charges 25,557 LCFS Credits charge 7,046 Total asset impairments and other charges $ 67,934 Inventory valuation provision 13,158 Total charges $ 81,092 Workforce Reduction and Related Charges As a result of the workforce reduction, severance costs of $2,757 were incurred in connection with employee terminations and $300 in stock-based compensation expense was incurred for the associated acceleration of certain stock awards. Impairments of Long-Lived Assets CEC: Asset Impairment Charges Due to the continued low global demand for compressors, and the decision to position CEC’s compressor business for industry consolidation with a potential strategic partner, the Company’s management determined that an impairment indicator was present for the long-lived assets of CEC. Recoverability was tested using future cash flow projections based on management’s long-term estimates of market conditions. Based on the results of this test, the sum of the undiscounted future cash flows was less than the carrying value of the CEC asset group. As a result, these long-lived assets were written down to their respective fair values, resulting in an impairment charge of $32,274 . Fair value was based on expected future cash flows using Level 3 inputs. The cash flows are those expected to be generated by market participants, discounted at an appropriate rate for the risks inherent in those cash flow projections. Station Closures and Related Charges During the third quarter of the year ended December 31, 2017, the Company decided to close 42 fueling stations by December 31, 2017, which were performing below management’s expectations based on volume and profitability levels. As a result, these station assets, which had an aggregate carrying value of $23,270 were written down to their respective fair values of $2,886 on an aggregate basis, resulting in a charge of $20,384 . The fair values of these assets were determined using the cost approach. In addition, certain of these station closures triggered related other charges totaling $5,173 , which consists of write-offs for any deferred losses, lease termination fees, and an increase in asset retirement obligations ("AROs"). Due to the closure of these stations, the Company's management assessed whether impairment indicators were present for the long-lived assets of the Company's other fueling stations. The Company determined there were no indicators of impairment present amongst the remaining fueling stations and no further steps were required for an impairment evaluation with respect to these stations. Inventory Valuation Provision As a result of the Company's evaluation process to minimize and eliminate underperforming station assets, the Company determined that $27,198 of certain station parts which were historically classified as construction in progress within Land, property, and equipment, net, were to be reclassified as Inventory in the consolidated balance sheets because they will primarily be used for stations to be sold. Subsequent to the reclassification, the Company calculated and recorded a lower of cost or market non-cash charge of $7,804 for these station parts. Additionally, in conjunction with its decision to seek a strategic partner for CEC, the Company incurred a lower of cost or market non-cash charge of $5,354 for the inventory of CEC. The aggregate amount of $13,158 is reported as Inventory valuation provision in the consolidated statements of operations for the year ended December 31, 2017. Cash Charges The following table summarizes the charges related to the foregoing that will be settled with cash payments and their related liability balances as of December 31, 2017 : Charges Cash Payments Made in the Year Ended December 31, 2017 Balance as of December 31, 2017 Employee severance $ 2,757 (2,757 ) $ — Lease termination fees and AROs for station closures 4,083 (70 ) 4,013 $ 6,840 (2,827 ) $ 4,013 LCFS Credits Charge The Company generates LCFS Credits when it sells RNG and conventional natural gas for use as a vehicle fuel and can sell and transfer these credits to third parties. The California Air Resources Board (“CARB”) restricted the Company’s ability to sell and transfer LCFS Credits during the third and fourth quarters of 2017, pending completion of an administrative review. The Company was, however, required to settle preexisting contractual obligations to transfer LCFS Credits to third parties by making cash payments totaling $7,046 , the equivalent value of the LCFS Credits the Company would have otherwise transferred to satisfy its obligations. These payments are reported in Asset impairments and other charges in the consolidated statements of operations for the year ended December 31, 2017 . In November 2017, CARB invalidated certain LCFS Credits the Company had generated in prior periods and released the restriction on the Company’s ability to sell and transfer LCFS Credits. The Company is disputing CARB’s invalidation of LCFS Credits.

Acquisitions and Divestitures

Acquisitions and Divestitures	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions and Divestitures	Acquisitions and Divestitures BP Transaction On February 27, 2017, Clean Energy Renewable Fuels ("Renewables"), a subsidiary of the Company, entered into an asset purchase agreement (the “APA”) with BP Products North America, Inc. (“BP”), pursuant to which Renewables agreed to sell to BP certain assets relating to its RNG production business (the “BP Transaction”), consisting of Renewables’ two existing RNG production facilities, Renewables’ interest in the RNG Ventures (as defined in Note 9 ) and Renewables’ third-party RNG supply contracts (the “Assets”). The BP Transaction was completed on March 31, 2017 for a sale price of $155,511 , plus BP assumed the obligations under the Canton Bonds (as defined in Note 11 ), which totaled $8,820 as of March 31, 2017. On March 31, 2017, BP paid Renewables $30,000 in cash and delivered to Renewables a promissory note with a principal amount of $123,487 (the "BP Note") which was paid in full on April 3, 2017. In addition, as a result of the determination of certain post-closing adjustments, BP paid Renewables an additional $2,010 on June 22, 2017. Pursuant to the APA, the valuation date of the BP Transaction was January 1, 2017, and as a result, the APA included certain adjustments to the purchase price to reflect a determination of the amount of cash accumulated by Renewables from the valuation date to the closing date, net of permitted cash outflows. Control of the Assets was not transferred until the BP Transaction was completed on March 31, 2017. Accordingly, the full operating results of Renewables are included in the consolidated statement of operations through the three months ended March 31, 2017. In addition, under the APA, BP is required, following the closing of the BP Transaction, to pay Renewables up to an additional $25,000 in cash over a five -year year period if certain performance criteria relating to the Assets are met. The Company satisfied the performance criteria for the first such period, which ended on December 31, 2017, and as a result, the Company recognized $772 which is included in the total gain on BP Transaction. The Company incurred $3,695 in transaction fees in connection with the BP Transaction, and subsequent to March 31, 2017, the Company paid $8,605 in cash and issued 770,269 shares of the Company's common stock, collectively valued at $1,964 , to holders of options to purchase membership units in Renewables. The net proceeds from the BP Transaction, net of $1,007 cash transferred to BP, were $142,190 . Following completion of the BP Transaction, Renewables and the Company are continuing to procure RNG from BP under a long-term supply contract and from other RNG suppliers, and resell such RNG through the Company's natural gas fueling infrastructure as Redeem, the Company's RNG vehicle fuel. The Company also collects royalties from BP on gas purchased from BP and sold as Redeem at the Company's stations, which royalty is in addition to any payment obligation of BP under the APA. The BP Transaction resulted in a total gain of $70,658 , which was recorded in Gain from sale of certain assets of subsidiary in the Company's consolidated statement of operations for the year ended December 31, 2017. Included in the determination of this gain amount is goodwill of $26,576 that was allocated to the disposed assets based on the relative fair values of the assets disposed and the portion of the reporting unit that was retained. The Company determined that the BP Transaction did not meet the definition of a discontinued operation because the disposal did not represent a strategic shift in the Company's strategy. SAFE&CEC S.r.l. On November 26, 2017, the Company, through its former subsidiary CEC, entered into an investment agreement with Landi Renzo S.p.A. (“LR”), an alternative fuels company based in Italy, pursuant to which the Company and LR agreed to combine their respective natural gas compressor subsidiaries, CEC and SAFE S.p.A, in a new company known as “SAFE&CEC S.r.l.” (such combination transaction, the “CEC Combination”). SAFE&CEC S.r.l. is focused on manufacturing, selling and servicing natural gas fueling compressors and related equipment for the global natural gas fueling market. Upon the closing of the CEC Combination, which occurred on December 29, 2017, the Company owns 49% of SAFE&CEC S.r.l. and LR owns 51% of SAFE&CEC S.r.l. The Company accounts for its interest in SAFE&CEC S.r.l. using the equity method of accounting because the Company does not control but has the ability to exercise significant influence over SAFE&CEC S.r.l.'s operations. The Company has an investment balance in SAFE&CEC S.r.l. of $27,883 as of December 31, 2017 . The fair value of the CEC Combination was determined using the income valuation approach. Under the income approach, we used a discounted cash flow model (“DCF”) in which cash flows anticipated over several periods, plus a terminal value at the end of that time horizon, are discounted to their present value using an appropriate expected discount rate. The discount rate used for cash flows reflects capital market conditions and the specific risks associated with the business. This valuation approach is considered a Level 3 fair value measurement. If actual results, market and economic conditions, including interest rates, and other factors are not consistent with management’s estimates and assumptions used in this calculation, the Company may be exposed to additional impairment losses. The CEC Combination resulted in a total loss of $6,465 , which was recorded in Loss from formation of equity method investment in the Company's consolidated statement of operations for the year ended December 31, 2017. The Company incurred working capital adjustments, funding for certain post-closing commitments, and transaction fees, of which $3,986 was unpaid and recorded in Accrued liabilities in the Company's consolidated balance sheet as of December 31, 2017. Included in this loss amount is goodwill of $3,578 that was allocated to the disposed assets based on the relative fair values of those assets and the portion of the reporting unit that was retained. Prior to the CEC Combination, CEC had pre-tax losses of $13,657 , $15,601 , and $45,126 for fiscal years 2015, 2016, and 2017, respectively. The Company determined that the CEC Combination did not meet the definition of a discontinued operation because the disposal did not represent a strategic shift that will have a major effect on the Company's operations and financial results.

Restricted Cash

Restricted Cash	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Restricted Cash	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. Short-term restricted cash as of December 31, 2016 and 2017 consisted of the following: December 31, 2016 December 31, 2017 Short-term restricted cash: Standby letters of credit $ 1,753 $ 1,127 Canton Bonds (see Note 11) 3,665 — Held in escrow 1,578 — Total short-term restricted cash $ 6,996 $ 1,127

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Available-for-sale securities are carried at fair value, inclusive of unrealized gains and losses. Unrealized gains and losses are included in other comprehensive income (loss) net of applicable income taxes. Gains or losses on sales of available-for-sale securities are recognized on the specific identification basis. All of the Company's short-term investments are classified as available-for-sale securities. The Company reviews available-for-sale securities for other-than-temporary declines in fair value below their cost basis each quarter and whenever events or changes in circumstances indicate that the cost basis of an asset may not be recoverable. This evaluation is based on a number of factors, including the length of time and the extent to which the fair value has been below its cost basis and adverse conditions related specifically to the security, including any changes to the credit rating of the security. As of December 31, 2017 , the Company believes its carrying values for its available-for-sale securities are properly recorded. Short-term investments as of December 31, 2016 consisted of the following: Amortized Gross Estimated Municipal bonds and notes $ 8,791 $ (4 ) $ 8,787 Corporate bonds 21,517 (7 ) 21,510 Certificate of deposits 43,421 — 43,421 Total short-term investments $ 73,729 $ (11 ) $ 73,718 Short-term investments as of December 31, 2017 consisted of the following: Amortized Gross Estimated Municipal bonds and notes $ 21,414 $ (49 ) $ 21,365 Zero coupon bonds 54,159 (33 ) 54,126 Corporate bonds 55,109 (40 ) 55,069 Certificate of deposits 10,902 — 10,902 Total short-term investments $ 141,584 $ (122 ) $ 141,462

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company follows the authoritative guidance for fair value measurements with respect to assets and liabilities that are measured at fair value on a recurring basis and non-recurring basis. Under the standard, fair value is defined as the exit price, or the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants, as of the measurement date. The standard also establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs market participants would use in valuing the asset or liability developed based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the factors market participants would use in valuing the asset or liability developed based upon the best information available in the circumstances. The hierarchy consists of the following three levels: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs (other than quoted prices) that are observable for the asset or liability, either directly or indirectly; Level 3 inputs are unobservable inputs for the asset or liability. Categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. As of December 31, 2017 , the Company's financial instruments consisted of available-for-sale securities, liability-classified warrants and debt instruments. The Company’s available-for-sale securities are classified within Level 2 because they are valued using the most recent quoted prices for identical assets in markets that are not active and quoted prices for similar assets in active markets. The liability-classified warrants are classified within Level 3 because the Company uses the Black-Scholes option pricing model to estimate the fair value based on inputs that are not observable in any market. The fair values of the Company's debt instruments approximated their carrying values as of December 31, 2016 and 2017 . See Note 11 for more information about the Company's debt instruments. There were no transfers of assets between Level 1, Level 2 or Level 3 of the fair value hierarchy as of December 31, 2016 and December 31, 2017 , respectively. The following tables provide information by level for assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2016 and December 31, 2017 , respectively: Description Balance at December 31, 2016 Level 1 Level 2 Level 3 Assets: Available-for-sale securities(1): Municipal bonds and notes $ 8,787 $ — $ 8,787 $ — Corporate bonds 21,510 — 21,510 — Certificate of deposits 43,421 — 43,421 — Liabilities: Warrants(2) 581 — — 581 Description Balance at December 31, 2017 Level 1 Level 2 Level 3 Assets: Available-for-sale securities(1): Municipal bonds and notes $ 21,365 $ 21,365 $ — Zero coupon bonds 54,126 54,126 — Corporate bonds 55,069 55,069 — Certificate of deposits 10,902 10,902 — Liabilities: Warrants(2) 536 — — 536 _______________________________________________________________________________ (1) Included in short-term investments in the consolidated balance sheets. See Note 5 for more information. (2) Included in accrued liabilities and other long-term liabilities in the consolidated balance sheets. The following table provides a reconciliation of the beginning and ending balances of items measured at fair value on a recurring basis as shown in the tables above that used significant unobservable inputs (Level 3): Liabilities: Warrants 2016 2017 Beginning Balance $ 632 $ 581 Gain included in earnings (51 ) (45 ) Ending Balance $ 581 $ 536 Non-Financial Assets In the year ended December 31, 2017, long-lived assets held and used with a carrying value of $59,367 were written down to their fair value of $6,709 , resulting in charges of $52,658 . The fair value of these assets was determined using Level 3 inputs. See Note 2 for more information.

Other Receivables

Other Receivables	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Other Receivables	Other Receivables Other receivables as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Loans to customers to finance vehicle purchases $ 7,416 $ 4,746 Accrued customer billings 4,308 10,072 Fuel tax credits 6,358 177 Other 3,852 4,240 Total other receivables $ 21,934 $ 19,235

Land, Property and Equipment

Land, Property and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Land, Property and Equipment	Land, Property and Equipment Land, property and equipment as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Land $ 2,858 $ 2,858 LNG liquefaction plants 94,634 94,634 RNG plants (1) 47,545 — Station equipment (2) 341,605 304,090 Trailers 54,985 70,906 Other equipment (2) 93,118 88,313 Construction in progress (2) (3) 117,662 74,905 752,407 635,706 Less accumulated depreciation (268,484 ) (268,401 ) Total land, property and equipment, net $ 483,923 $ 367,305 (1) The RNG plants were sold in the BP Transaction (see Note 3 for more information). (2) Certain of these assets were written down during the year ended December 31, 2017 (see Note 2 for more information). (3) During the year ended December 31, 2017 , $19,394 in station parts were reclassified from construction in progress within Land, property, and equipment, net, into Inventory in the consolidated balance sheets because they will primarily be used for stations to be sold (see Note 2 for more information). Included in the land, property and equipment are capitalized software costs of $25,728 and $26,003 as of December 31, 2016 and 2017 , respectively. The accumulated amortization of the capitalized software costs is $17,237 and $18,737 as of December 31, 2016 and 2017 , respectively. The Company recorded $3,053 , $3,444 and $4,382 of amortization expense related to the capitalized software costs during the years ended December 31, 2015 , 2016 and 2017 , respectively. As of December 31, 2016 and 2017 , $4,053 and $4,377 , respectively, are included in accounts payable and accrued liabilities balances, which amounts are related to purchases of property and equipment. These amounts are excluded from the consolidated statements of cash flows as they are non-cash investing activities.

Investments in Other Entities a

Investments in Other Entities and Noncontrolling Interest in a Subsidiary	12 Months Ended
	Dec. 31, 2017
Investments, All Other Investments [Abstract]
Investments in Other Entities and Noncontrolling Interest in a Subsidiary	Investments in Other Entities and Noncontrolling Interest in a Subsidiary SAFE&CEC S.r.l. On December 29, 2017, the Company obtained a 49% ownership interest in SAFE&CEC S.r.l. See Note 3 for more information. RNG Ventures In November 2016, Renewables entered into agreements to form joint ventures with Aria Energy Operating LLC ("Aria"), a developer of RNG production facilities, to develop RNG production facilities at a Republic Services landfill in Oklahoma City, Oklahoma and an Advanced Disposal landfill near Atlanta, Georgia. These joint ventures are referred to as the "RNG Ventures." Renewables' interest in the RNG Ventures was transferred to BP upon completion of the BP Transaction (see Note 3 for more information); however, Renewables retained the right to purchase 100% of the RNG that will be produced by these facilities for the vehicle fuels market. The Company accounted for its interest in the RNG Ventures using the equity method of accounting as the Company had the ability to exercise significant influence over these operations. The Company had an investment balance of $833 and $0 in the RNG Ventures as of December 31, 2016 and 2017 , respectively. MCEP On September 16, 2014, the Company formed a joint venture with Mansfield Ventures LLC (“Mansfield Ventures”) called Mansfield Clean Energy Partners LLC ("MCEP"), which is designed to provide natural gas fueling solutions to bulk fuel haulers in the United States. The Company and Mansfield Ventures each have a 50% ownership interest in MCEP. The Company accounts for its interest in MCEP using the equity method of accounting as the Company has the ability to exercise significant influence over MCEP's operations. The Company recorded a loss from this investment of $815 , $22 and $131 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Additionally, during the year ended December 31, 2016, the Company received a return of capital of $3,031 with no change in ownership interest. The Company had an investment balance of $1,642 and $1,512 in MCEP as of December 31, 2016 and 2017 , respectively. NG Advantage On October 14, 2014, the Company entered into a Common Unit Purchase Agreement (“UPA”) with NG Advantage, LLC (“NG Advantage”) for a 53.3% controlling interest in NG Advantage, which increased subsequent to December 31, 2017 (see Note 19 for more information). NG Advantage is engaged in the business of transporting CNG in high-capacity trailers to industrial and institutional energy users, such as hospitals, food processors, manufacturers and paper mills that do not have direct access to natural gas pipelines. The Company viewed the acquisition as a strategic investment in the expansion of the Company’s initiative to deliver natural gas to industrial and institutional energy users. The results of NG Advantage’s operations have been included in the Company’s consolidated financial statements since October 14, 2014. On July 14, 2017, the Company contributed to NG Advantage all of its right, title and interest in and to a CNG fueling station located in Milton, Vermont. The Company had purchased this CNG station from NG Advantage in October 2014 in connection with the UPA, and at that time, the Company entered into a lease agreement with NG Advantage to lease the station back to NG Advantage. This lease agreement was terminated contemporaneously with the contribution of the station to NG Advantage in July 2017. As consideration for the contribution, NG Advantage issued to the Company Series A Preferred Units with an aggregate value of $7,500 . The Series A Preferred Units provide for an accrued return in the event of a liquidation event with respect to NG Advantage and will convert into common units of NG Advantage if and when it completes a future equity financing that satisfies certain specified conditions; however, the Series A Preferred Units do not, in themselves, increase the Company's controlling interest in NG Advantage. As a result, immediately following the contribution, the Company's controlling interest in NG Advantage remained at 53.3% . The Company recorded a loss from the noncontrolling interest in NG Advantage of $1,216 , $1,571 and $2,154 for the years ended December 31, 2015 , 2016 and 2017 , respectively. The noncontrolling interest was $24,822 and $22,668 as of December 31, 2016 and 2017 , respectively.

Accrued Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Accrued alternative fuels incentives (1) 9,840 2,954 Accrued employee benefits 4,317 2,378 Accrued interest 1,849 1,486 Accrued gas and equipment purchases 11,657 8,722 Accrued property and other taxes 4,572 4,582 Salaries and wages 12,293 8,363 Other (2) 8,073 13,783 Total accrued liabilities $ 52,601 $ 42,268 (1) Includes the amount of RINs and LCFS Credits and, as of December 31, 2016, the amount of AFTC payable to third parties. No AFTC amounts were accrued as of December 31, 2017 because, as of that date, the AFTC had expired (subsequent to December 31, 2017, however, the AFTC was reinstated for vehicle fuel sales made from January 1, 2017 through December 31, 2017). See Notes 1 and 19 for more information about AFTC. (2) The amount as of December 31, 2017 includes lease termination fees and AROs related to closure of certain fueling stations (see Note 2 for more information) and working capital adjustments, funding for certain commitments, and transaction fees incurred as a result of the CEC Combination (see Note 3 for more information).

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	Debt Debt and capital lease obligations as of December 31, 2016 and 2017 consisted of the following and are further discussed below: December 31, 2016 Principal Balances Unamortized Debt Financing Costs Balance, Net of Financing Costs 7.5% Notes (1) $ 150,000 274 $ 149,726 5.25% Notes 110,450 1,088 $ 109,362 PlainsCapital Bank 23,500 — $ 23,500 Canton Bonds 9,520 373 $ 9,147 Capital lease obligations 6,028 — $ 6,028 Other debt 14,850 237 $ 14,613 Total debt and capital lease obligations 314,348 1,972 312,376 Less amounts due within one year (6,126 ) (183 ) (5,943 ) Total long-term debt and capital lease obligations $ 308,222 $ 1,789 $ 306,433 December 31, 2017 Principal Balances Unamortized Debt Financing Costs Balance, Net of Financing Costs 7.5% Notes (1) 125,000 131 $ 124,869 5.25% Notes 110,450 454 $ 109,996 NG Advantage debt 17,185 259 $ 16,926 Capital lease obligations 7,054 — $ 7,054 Other debt 1,242 — $ 1,242 Total debt and capital lease obligations 260,931 844 260,087 Less amounts due within one year (140,223 ) (524 ) (139,699 ) Total long-term debt and capital lease obligations $ 120,708 $ 320 $ 120,388 (1) Includes $65,000 and $0 in principal amount held by Mr. Pickens as of December 31, 2016 and December 31, 2017 , respectively, which is classified as “Long-term debt, related party” on the consolidated balance sheets. See the description below for more information. The following is a summary of the aggregate maturities of debt and capital lease obligations for each of the yearly periods subsequent to December 31, 2017 : 2018 2019 2020 2021 2022 Thereafter 7.5% Notes (1) 25,000 50,000 50,000 — — — 5.25% Notes 110,450 — — — — — NG Advantage debt 3,483 3,242 3,413 3,055 2,665 1,327 Capital lease obligations 1,072 1,021 925 871 917 2,248 Other debt 218 229 240 247 215 93 Total $ 140,223 $ 54,492 $ 54,578 $ 4,173 $ 3,797 $ 3,668 7.5% Notes On July 11, 2011, the Company entered into a loan agreement (the "CHK Agreement") with Chesapeake NG Ventures Corporation ("Chesapeake"), an indirect wholly owned subsidiary of Chesapeake Energy Corporation, whereby Chesapeake agreed to purchase from the Company up to $150,000 of debt securities pursuant to the issuance of three convertible promissory notes over a three -year period, each having a principal amount of $50,000 (each a "CHK Note" and collectively the "CHK Notes" and, together with the CHK Agreement and other transaction documents, the "CHK Loan Documents"). The first CHK Note was issued on July 11, 2011 and the second CHK Note was issued on July 10, 2012. On June 14, 2013 (the "Transfer Date"), Mr. Pickens and Green Energy Investment Holdings, LLC ("GEIH"), an affiliate of Leonard Green & Partners, L.P. (collectively, the "Buyers") and Chesapeake entered into a note purchase agreement ("Note Purchase Agreement") pursuant to which Chesapeake sold the outstanding CHK Notes (the "Sale") to the Buyers. Chesapeake assigned to the Buyers all of its right, title and interest under the CHK Loan Documents (the "Assignment") and each Buyer severally assumed all of the obligations of Chesapeake under the CHK Loan Documents arising after the Sale and the Assignment including, without limitation, the obligation to advance an additional $50,000 to the Company in June 2013 (the "Assumption"). The Company also entered into the Note Purchase Agreement for the purpose of consenting to the Sale, the Assignment and the Assumption. Contemporaneously with the execution of the Note Purchase Agreement, the Company entered into a loan agreement with each Buyer (collectively, the "Amended Agreements"). The Amended Agreements have the same terms as the CHK Agreement, other than changes to reflect the new holders of the CHK Notes. Immediately following execution of the Amended Agreements, the Buyers delivered $50,000 to the Company in satisfaction of the funding requirement they had assumed from Chesapeake (the "2013 Advance"). In addition, the Company canceled the existing CHK Notes and issued replacement notes and the Company also issued notes to the Buyers in exchange for the 2013 Advance (the replacement notes and the notes issued in exchange for the 2013 Advance are referred to herein as the " 7.5% Notes"). The 7.5% Notes have the same terms as the original CHK Notes, other than changes to reflect their different holders. They bear interest at the rate of 7.5% per annum and are convertible at the option of the holder into shares of the Company's common stock at a conversion price of $15.80 per share (the " 7.5% Notes Conversion Price"). Upon written notice to the Company, each holder of a 7.5% Note has the right to exchange all or any portion of the principal and accrued and unpaid interest under its 7.5% Notes for shares of the Company's common stock at the 7.5% Notes Conversion Price. Additionally, subject to certain restrictions, the Company can force conversion of each 7.5% Note into shares of its common stock if, following the second anniversary of the issuance of a 7.5% Note, such shares trade at a 40% premium to the 7.5% Notes Conversion Price for at least 20 trading days in any consecutive 30 trading day period. The entire principal balance of each 7.5% Note is due and payable seven years following its issuance and the Company may repay each 7.5% Note at maturity in shares of its common stock (provided that the Company may not issue more than 13,993,630 shares of its common stock to holders of 7.5% Notes) or cash. All of the shares issuable upon conversion of the 7.5% Notes have been registered for resale by their holders pursuant to a registration statement that has been filed with and declared effective by the Securities and Exchange Commission. The Amended Agreements provide for customary events of default which, if any of them occurs, would permit or require the principal of, and accrued interest on, the 7.5% Notes to become, or to be declared, due and payable. No events of default under the 7.5% Notes had occurred as of December 31, 2017 . On August 27, 2013, GEIH transferred $16,800 in principal amount of its 7.5% Notes to third parties. On February 9, 2017, the Company purchased from Mr. Pickens, his 7.5% Note due July 2018 having an outstanding principal amount of $25,000 for a cash purchase price of $21,750 . The Company's repurchase of this 7.5% Note resulted in a gain of $3,191 for the year ended December 31, 2017. On February 21, 2017, GEIH transferred $11,800 in principal amount of its 7.5% Notes to third parties. On November 17, 2017, Mr. Pickens transferred all remaining $40,000 in principal amount of his 7.5% Notes to third parties. As a result of the foregoing transactions, as of December 31, 2017 , (i) GEIH held 7.5% Notes in an aggregate principal amount of $68,200 and (ii) other third parties held 7.5% Notes in an aggregate principal amount of $56,800 . SLG Notes On August 24, 2011, the Company entered into convertible note purchase agreements (each, an "SLG Agreement" and collectively the "SLG Agreements") with each of Springleaf Investments Pte. Ltd., a wholly owned subsidiary of Temasek Holdings Pte. Ltd., Lionfish Investments Pte. Ltd., an investment vehicle managed by Seatown Holdings International Pte. Ltd., and Greenwich Asset Holding Ltd., a wholly owned subsidiary of RRJ Capital Master Fund I, L.P. (each, a "Purchaser" and collectively, the "Purchasers"), pursuant to which the Purchasers agreed to purchase from the Company $150,000 of 7.5% convertible promissory notes due in August 2016 (each a "SLG Note" and collectively the "SLG Notes"). The transaction closed and the SLG Notes were issued on August 30, 2011. On March 1, 2012, Springleaf Investments Pte Ltd. transferred $24,000 in principal amount of the SLG Notes to Baytree Investments (Mauritius) Pte Ltd. On February 29, 2016, the Company prepaid in cash an aggregate of $60,000 in principal amount and $1,812 in accrued and unpaid interest owed under the SLG Notes. On July 14, 2016, the Company exchanged the outstanding principal amount of the SLG Notes, totaling $85,000 , and all accrued and unpaid interest thereon, totaling $248 , for an aggregate of 14,000,000 shares of the Company's common stock and $38,155 in cash. The Company recognized a loss of $891 for the year ended December 31, 2016 related to the exchange of the SLG Notes for the Company's common stock. The repurchased and exchanged SLG Notes have been surrendered and canceled in full and the Company has no further obligations under the SLG Notes. 5.25% Notes In September 2013, the Company completed a private offering of $250,000 in principal amount of 5.25% Convertible Senior Notes due 2018 (the " 5.25% Notes") and entered into an indenture governing the 5.25% Notes (the "Indenture"). The net proceeds from the sale of the 5.25% Notes after the payment of certain debt issuance costs of $7,805 were $242,195 . The Company has used the net proceeds from the sale of the 5.25% Notes to fund capital expenditures and for general corporate purposes. The 5.25% Notes bear interest at a rate of 5.25% per annum, payable semi- annually in arrears on October 1 and April 1 of each year, beginning on April 1, 2014. The 5.25% Notes will mature on October 1, 2018, unless purchased, redeemed or converted prior to such date in accordance with their terms and the terms of the Indenture. Holders may convert their 5.25% Notes, at their option, at any time prior to the close of business on the business day immediately preceding the maturity date of the 5.25% Notes. Upon conversion, the Company will deliver a number of shares of its common stock, per $1 principal amount of 5.25% Notes, equal to the conversion rate then in effect (together with a cash payment in lieu of any fractional shares). The initial conversion rate for the 5.25% Notes is 64.1026 shares of the Company's common stock per $1 principal amount of 5.25% Notes (which is equivalent to an initial conversion price of approximately $15.60 per share of the Company's common stock). The conversion rate is subject to adjustment upon the occurrence of certain specified events as described in the Indenture. Upon the occurrence of certain corporate events prior to the maturity date of the 5.25% Notes, the Company will, in certain circumstances, in addition to delivering the number of shares of the Company's common stock deliverable upon conversion of the 5.25% Notes based on the conversion rate then in effect (together with a cash payment in lieu of any fractional shares), pay holders that convert their 5.25% Notes, a cash make-whole payment in an amount as described in the Indenture. The Company may, at its option, irrevocably elect to settle its obligation to pay any such make-whole payment in shares of its common stock instead of in cash. The amount of any make-whole payment, whether it is settled in cash or in shares of the Company's common stock upon the Company's election, will be determined based on the date on which the corporate event occurs or becomes effective and the stock price paid (or deemed to be paid) per share of the Company's common stock in the corporate event, as described in the Indenture. The Company may not redeem the 5.25% Notes prior to October 5, 2016. On or after October 5, 2016, the Company may, at its option, redeem for cash all or any portion of the 5.25% Notes if the closing sale price of the Company's common stock for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which notice of redemption is provided, exceeds 160% of the conversion price on each applicable trading day. In the event of the Company's redemption of the 5.25% Notes, the redemption price will equal 100% of the principal amount of the 5.25% Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. No sinking fund is provided for in the 5.25% Notes. If the Company undergoes a fundamental change (as defined in the Indenture) prior to the maturity date of the 5.25% Notes, subject to certain conditions as described in the Indenture, holders may require the Company to purchase, for cash, all or any portion of their 5.25% Notes at a repurchase price equal to 100% of the principal amount of the 5.25% Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change purchase date. The Indenture contains customary events of default with customary cure periods, including, without limitation, failure to make required payments or deliveries of shares of the Company’s common stock when due under the Indenture, failure to comply with certain covenants under the Indenture, failure to pay when due or acceleration of certain other indebtedness of the Company or certain of its subsidiaries, and certain events of bankruptcy and insolvency of the Company or certain of its subsidiaries. The occurrence of an event of default under the Indenture will allow either the trustee or the holders of at least 25% in principal amount of the then-outstanding 5.25% Notes to accelerate, or upon an event of default arising from certain events of bankruptcy or insolvency of the Company, will automatically cause the acceleration of, all amounts due under the 5.25% Notes. No events of default under the 5.25% Notes had occurred as of December 31, 2017 . The 5.25% Notes are senior unsecured obligations of the Company and rank senior in right of payment to the Company's future indebtedness that is expressly subordinated in right of payment to the 5.25% Notes; equal in right of payment to the Company's unsecured indebtedness that is not so subordinated; effectively junior to any of the Company's secured indebtedness to the extent of the value of the assets securing such indebtedness; and structurally junior to all indebtedness (including trade payables) of the Company's subsidiaries. For the year ended December 31, 2016, the Company paid an aggregate of $84,344 in cash to repurchase and retire $114,550 in aggregate principal amount of the 5.25% Notes, together with $1,546 in accrued and unpaid interest thereon. Additionally, pursuant to a privately negotiated exchange agreement with certain holders of the 5.25% Notes, on May 4, 2016, the Company issued 6,265,829 shares of its common stock in exchange for an aggregate principal amount of $25,000 of 5.25% Notes held by such holders and accrued and unpaid interest thereon. The value of the shares of the Company's common stock issued to the holders of the 5.25% Notes in the exchange has been excluded from the Company's consolidated statements of cash flows as it is a non-cash financing activity. The Company's repurchase and exchange of 5.25% Notes for the year ended December 31, 2016 resulted in a total gain of $35,239 recorded during the period. All repurchased and exchanged 5.25% Notes have been surrendered to the trustee for such notes and canceled in full and the Company has no further obligations under such notes. PlainsCapital Bank Credit Facility On February 29, 2016, the Company entered into a Loan and Security Agreement (“Plains LSA”) with PlainsCapital Bank (“Plains”), pursuant to which Plains agreed to lend the Company up to $50,000 on a revolving basis from time to time for a term of one year (the “Credit Facility”). All amounts advanced under the Credit Facility were due and payable on February 28, 2017. Simultaneously, the Company drew $50,000 under this Credit Facility, which the Company repaid in full on August 31, 2016. On October 31, 2016, the Plains LSA was amended solely to extend the Credit Facility's maturity date from February 28, 2017 to September 30, 2018. On December 22, 2016, the Company drew $23,500 under the Credit Facility, which the Company repaid in full on March 31, 2017. As a result, the Company had no amounts outstanding under the Credit Facility as of December 31, 2017 . The Credit Facility is evidenced by a promissory note the Company issued on February 29, 2016 in favor of Plains (the “Plains Note”). Interest on the Plains Note is payable monthly and accrues at a rate equal to the greater of (i) the then-current LIBOR rate plus 2.30% or (ii) 2.70% . As collateral security for the prompt payment in full when due of the Company's obligations to Plains under the Plains LSA and the Plains Note, the Company pledged to and granted Plains a security interest in all of its right, title and interest in the cash and corporate and municipal bonds rated AAA, AA or A by Standard & Poor’s Rating Services that the Company holds in an account at Plains. In connection with such pledge and security interest granted under the Credit Facility, on February 29, 2016, the Company entered into a Pledged Account Agreement with Plains and PlainsCapital Bank - Wealth Management and Trust (the “Pledge Agreement” and collectively with the Plains LSA and the Plains Note, the “Plains Loan Documents”).The Plains Loan Documents include certain covenants of the Company and also provide for customary events of default, which, if any of them occurs, would permit or require, among other things, the principal of, and accrued interest on, the Credit Facility to become, or to be declared, due and payable. Events of default under the Plains Loan Documents include, among others, the occurrence of certain bankruptcy events, the failure to make payments when due under the Plains Note and the transfer or disposal of the collateral under the Plains LSA. No events of default under the Plains Loan Documents had occurred as of December 31, 2017 . Canton Bonds On March 19, 2014, Canton Renewables LLC ("Canton"), a former subsidiary of the Company, completed the issuance of Solid Waste Facility Limited Obligation Revenue Bonds (Canton Renewables, LLC—Sauk Trail Hills Project) Series 2014 in the aggregate principal amount of $12,400 (the "Canton Bonds"). The Canton Bonds were issued by the Michigan Strategic Fund (the "Issuer") and the proceeds of the issuance were loaned by the Issuer to Canton pursuant to a loan agreement that became effective on March 19, 2014. On March 31, 2017, Canton was sold to BP in the BP Transaction (see Note 3 ). As a result, the Canton Bonds became the obligation of BP as of such date. NG Advantage Debt On May 12, 2016 and January 24, 2017, respectively, NG Advantage entered into a Loan and Security Agreement (the “Commerce LSA”) with Commerce Bank & Trust Company (“Commerce”), pursuant to which Commerce agreed to lend NG Advantage $6,300 and $6,150 , respectively. The proceeds were primarily used to fund the purchases of CNG trailers and equipment. Interest and principal for both loans are payable monthly in 84 equal monthly installments at an annual rate of 4.41% and 5.0% , respectively. As collateral security for the prompt payment in full when due of NG Advantage's obligations to Commerce under the Commerce LSA, NG Advantage pledged to and granted Commerce a security interest in all of its right, title and interest in the CNG trailers and equipment purchased with the proceeds received under the Commerce LSA. On November 30, 2016, NG Advantage entered into a Loan and Security Agreement (the "Wintrust LSA") with Wintrust Commercial Finance (“Wintrust”), pursuant to which Wintrust agreed to lend NG Advantage $4,695 . The proceeds were primarily used to fund the purchases of CNG trailers and equipment. Interest and principal is payable monthly in 72 equal monthly installments at an annual rate of 5.17% . As collateral security for the prompt payment in full when due of NG Advantage's obligations to Wintrust under the Wintrust LSA, NG Advantage pledged to and granted Wintrust a security interest in all of its right, title and interest in the CNG trailers and equipment purchased with the proceeds received under the Wintrust LSA. NG Advantage has other debt for trailers and equipment due at various dates through 2024 bearing interest at rates up to 6.01% , with weighted -average interest rates of 5.51% and 4.87% , and outstanding principal balance of $2,598 and $9,207 as of December 31, 2016 and December 31, 2017 , respectively. Other Debt The Company has other debt due at various dates through 2023 bearing interest at rates up to 5.02% and with a weighted -average interest rate of 5.72% and 4.79% as of December 31, 2016 and 2017 , respectively. December 2015 Termination of GE Credit Agreement On November 7, 2012, the Company, through two wholly owned subsidiaries (the "Borrowers"), entered into a credit agreement ("Credit Agreement") with General Electric Capital Corporation ("GE"). Pursuant to the Credit Agreement, GE agreed to loan to the Borrowers up to an aggregate of $200,000 to finance the development, construction and operation of two LNG plants (individually a "Project" and together the "Projects"). Concurrently with the execution of the Credit Agreement, the Company issued to GE a warrant to purchase up to 5,000,000 shares of its common stock at a price of $0.01 per share (the "GE Warrant"). On December 31, 2015, the Company terminated the Credit Agreement and all related documents except for the GE Warrant, which effectively became exercisable for only 1,000,000 shares because the vesting conditions relating to the other 4,000,000 shares subject to the GE Warrant could not occur following the termination. See Note 12 for more information about the GE Warrant. No amounts had been borrowed by the Borrowers under the Credit Agreement as of its termination. As a result of the termination, all related unamortized deferred financing costs that were to be amortized to interest expense in future periods have been removed from the balance sheet and a non-cash charge totaling $54,925 was recorded in interest expense in the fourth quarter of the year ended December 31, 2015.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Stockholders' Equity	Stockholders' Equity Authorized Shares The Company's certificate of incorporation authorizes the issuance of two classes of capital stock designated as common stock and preferred stock, each having $0.0001 par value per share. As of December 31, 2017 , the Company was authorized to issue 225,000,000 shares, of which 224,000,000 shares are designated common stock and 1,000,000 shares are designated preferred stock. Dividend Provisions The Company did not declare or pay any dividends during the years ended December 31, 2015 , 2016 or 2017 . Voting Rights Each holder of common stock has the right to one vote per share owned on matters presented for stockholder action. Issuance of Common Stock and Warrants Series I Warrants In November 2008, the Company issued to certain investors 4,419,192 Series I Warrants to purchase up to 3,314,394 shares of common stock. The Series I Warrants became exercisable beginning six months from the date of issuance, had a term of seven years from the date they became exercisable, and carried an exercise price of $12.54 per share. All outstanding Series I Warrants expired in April 2016. GE Warrant Concurrently with the execution of the Credit Agreement on November 7, 2012, the Company issued to GE the GE Warrant, which entitled GE to purchase up to 5,000,000 shares of the Company's common stock at a price per share of $0.01 . The Company terminated the Credit Agreement on December 31, 2015 and as a result, 4,000,000 shares subject to the GE Warrant could not become exercisable because the vesting conditions relating to these shares could not occur following such termination. On October 4, 2016, the holders of the GE Warrant exercised the warrant to purchase the 1,000,000 shares of common stock that were vested and exercisable thereunder pursuant to the cashless exercise provisions thereof, which resulted in the Company's issuance of 997,740 shares of common stock to such holders. Following such exercise, the GE Warrant has been surrendered and canceled in full and the Company has no further obligations under the GE Warrant. The Company measured the fair value of the original 5,000,000 shares subject to the GE Warrant at $56,158 and recorded the amount in additional paid-in-capital and other long-term assets as a deferred financing cost. The fair value of the 1,000,000 shares that remained exercisable under the GE Warrant following the termination of the Credit Agreement were being amortized over the estimated term of the Credit Agreement on the straight-line basis. The issuance of the GE Warrant is not included in the consolidated statements of cash flows as it is a non-cash financing activity. In connection with the termination of the Credit Agreement, all related unamortized deferred financing costs that were to be amortized to interest expense in future periods have been eliminated from the consolidated balance sheets in full through a non-cash charge to the consolidated statement of operations, reported in interest expense, of $54,925 in the fourth quarter of 2015. At-The-Market Offering Program On May 31, 2017, the Company terminated its equity distribution agreement (the “Sales Agreement”) with Citigroup Global Markets Inc. (“Citigroup”), as sales agent and/or principal. The Sales Agreement was terminable at will upon written notification by the Company with no penalty. Pursuant to the Sales Agreement, the Company was entitled to issue and sell, from time to time, through or to Citigroup, shares of its common stock having an aggregate offering price of up to $200,000 in an “at-the-market” offering program (the “ATM Program”). The ATM Program commenced on November 11, 2015 when the Company and Citigroup entered into the original equity distribution agreement, which was amended and restated on September 9, 2016 and again on December 21, 2016 prior to its termination. The following table summarizes the activity under the ATM Program for the periods presented: Year ended December 31, Year ended December 31, Year ended December 31, (in 000s, except per-share amounts) 2015 2016 2017 Gross proceeds $ 6,943 $ 103,591 $ 10,767 Fees and issuance costs 493 2,612 311 Net proceeds $ 6,450 $ 100,979 $ 10,456 Shares issued 1,561,902 31,064,434 3,802,500 Other As of December 31, 2017 , third parties held outstanding warrants, which expire in 2020, to purchase equity interests in NG Advantage. Such warrants allow the purchase of up to 127,200 NG Advantage common units and are accounted for as liability-classified warrants. The fair value was $581 and $536 as of December 31, 2016 and 2017 , respectively (see Note 6 for more information) and the gain (loss) from the change in fair value was $69 , $(21) and $45 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Stock-Based Compensation The following table summarizes the compensation expense and related income tax benefit related to the Company's stock-based compensation arrangements recognized in the consolidated statements of operations during the periods presented: Years Ended December 31, 2015 2016 2017 Stock-based compensation expense, net of $0 tax in 2015, 2016 and 2017 (1) $ 10,779 $ 8,092 $ 8,423 (1) $300 of stock-based compensation expense for the year ended December 31, 2017 is recorded in asset impairments and other charges in the consolidated statements of operations and is reported in non-cash portion of asset impairments and other charges in the consolidated statements of cash flows. See Note 2 for more information. Equity Incentive Plans In December 2002, the Company adopted its 2002 Stock Option Plan ("2002 Plan"). In December 2006, the Company adopted its 2006 Equity Incentive Plan ("2006 Plan"), which became effective on May 24, 2007, the date the Company completed its initial public offering of common stock. The 2002 Plan became unavailable for new awards upon the effectiveness of the 2006 Plan, at which time unissued awards under the 2002 Plan became available for grant under the 2006 Plan. In May 2016, the Company adopted its 2016 Performance Incentive Plan ("2016 Plan"), which became effective on May 26, 2016, the date of approval of the 2016 Plan by the Company's stockholders. The 2006 Plan became unavailable for new awards upon the effectiveness of the 2016 Plan. Unissued awards under the 2002 and 2006 Plans are not available for future grant under the 2016 Plan. If any outstanding award under the 2002 Plan or 2006 Plan expires or is canceled, the shares allocable to the unexercised portion of that award will be added to the share reserve under the 2016 Plan and will be available for grant under the 2016 Plan. As of December 31, 2017 , the Company had 4,823,956 shares available for future grant under the 2016 Plan. Stock Options The following table summarizes the Company's stock option activity: Number of Weighted Weighted Aggregate Options Outstanding as of December 31, 2014 11,486,301 $ 11.91 Granted 1,415,200 5.39 Exercised (608,279 ) 2.96 Forfeited or Expired (805,284 ) 14.04 Options Outstanding as of December 31, 2015 11,487,938 $ 11.44 Granted 284,750 3.63 Exercised — — Forfeited or Expired (304,892 ) 11.30 Options Outstanding as of December 31, 2016 11,467,796 $ 11.25 Granted 1,139,500 2.83 Exercised — — Forfeited or Expired (3,993,442 ) 12.34 Options Outstanding as of December 31, 2017 8,613,854 $ 9.62 4.65 — Options Exercisable as of December 31, 2017 7,118,956 $ 10.91 3.81 — Options Vested and Expected to Vest as of December 31, 2017 8,613,854 $ 9.62 4.65 — As of December 31, 2017 , there was $2,076 of total unrecognized compensation cost related to unvested shares underlying outstanding stock options. That cost is expected to be expensed over a remaining weighted average period of 1.51 years. The total fair value of shares vested during the year ended December 31, 2017 was $2,463 . The intrinsic value of all stock options exercised during the year ended December 31, 2015 , 2016 and 2017 was $2,197 , $0 and $0 respectively. The fair value of each stock option granted was estimated as of the date of grant using the Black-Scholes option pricing model and using the following assumptions: Years Ended December 31, 2015 2016 2017 Dividend yield 0.0% 0.0% 0.0% Expected volatility 59.2% to 72.0% 61.1% to 70.8% 63.61% Risk-free interest rate 1.7% to 1.8% 1.2% to 2.0% 2.05% Expected life in years 6.0 6.0 6.0 The weighted-average grant date fair values of stock options granted during the years ended December 31, 2015 , 2016 and 2017 , were $ 3.29 , $2.30 and $1.67 , respectively. The volatility amounts used were estimated based on the Company's historical and implied volatility of its traded options. The expected lives used were based on historical exercise periods and the Company's anticipated exercise periods for its outstanding stock options. The risk free interest rates used were based on the U.S. Treasury yield curve for the expected life of the stock options at the time of grant. The Company recorded $5,195 , $2,561 and $2,213 of stock option expense during the years ended December 31, 2015 , 2016 and 2017 , respectively. The Company has not recorded any tax benefit related to its stock option expense. Market-Based Performance Restricted Stock Units The Company granted 2,034,500 market-based performance restricted stock units ("Market-Based RSUs") to certain key employees during 2012 and 2014 . A holder of Market-Based RSUs will receive one share of the Company's common stock for each Market-Based RSU held if (x) between two years and four years from the date of grant of the Market-Based RSU, the closing price of the Company's common stock equals or exceeds, for twenty consecutive trading days, 135% of the closing price of the Company's common stock on the Market-Based RSU grant date (the "Stock Price Condition") and (y) the holder is employed by the Company at the time the Stock Price Condition is satisfied. If the Stock Price Condition is not satisfied prior to four years from the date of grant, the Market-Based RSUs are automatically forfeited. As a result, as of December 31, 2017 , Market-Based RSUs granted in January and May 2012 and entitling the holders to receive 1,700,000 shares of the Company’s common stock had been forfeited for failure to satisfy the applicable Stock Price Condition. The Market-Based RSUs are subject to the terms and conditions of the 2006 Plan and a Notice of Grant of Restricted Stock Unit and Restricted Stock Unit Agreement. The following table summarizes the Company's Market-Based RSU activity: Number of Weighted Weighted RSU Outstanding as of December 31, 2014 1,769,000 $ 10.67 Granted — — Vested — — Forfeited or Expired — — RSU Outstanding as of December 31, 2015 1,769,000 $ 10.67 Granted — — Vested — — Forfeited or Expired (1,340,000 ) 11.44 RSU Outstanding as of December 31, 2016 429,000 $ 8.26 Granted — — Vested — — Forfeited or Expired (94,500 ) 8.26 RSU Outstanding and Unvested as of December 31, 2017 334,500 $ 8.26 0.09 RSU Expected to Vest as of December 31, 2017 — — 0.00 As of December 31, 2017 , there was $0 of total unrecognized compensation cost related to unvested shares underlying outstanding Market-Based RSUs. The Company recorded $1,770 , $169 and $0 of expense during the years ended December 31, 2015 , 2016 and 2017 , respectively, related to the Market-Based RSUs. The Company has not recorded any tax benefit related to its Market-Based RSU expense. Service-Based Restricted Stock Units The Company has granted service-based restricted stock units ("Service-Based RSUs") to key employees that vest annually over the three years following the date of grant at a rate of 34% , 33% and 33% , respectively, if the holder is in service to the Company at each vesting date. The Service-Based RSUs are subject to the terms and conditions of the 2006 and 2016 Plans and a Notice of Grant of Restricted Stock Unit and Restricted Stock Unit Agreement. The following table summarizes the Company's Service-Based RSU activity: Number of Weighted Weighted RSU Outstanding as of December 31, 2014 822,752 $ 5.82 Granted 1,167,750 5.38 Vested (283,726 ) 5.94 Forfeited or expired (56,000 ) 5.57 RSU Outstanding as of December 31, 2015 1,650,776 $ 5.50 Granted 850,125 3.63 Vested (726,687 ) 5.53 Forfeited or expired (130,910 ) 4.91 RSU Outstanding as of December 31, 2016 1,643,304 $ 4.56 Granted 2,835,331 1.36 Vested (2,840,584 ) 1.97 Forfeited or expired (139,976 ) 4.69 RSU Outstanding and Unvested as of December 31, 2017 1,498,075 $ 3.41 0.86 RSU Expected to Vest as of December 31, 2017 1,498,075 $ 3.41 0.86 As of December 31, 2017 , there was $3,145 of total unrecognized compensation cost related to unvested shares underlying outstanding Service-Based RSUs. That cost is expected to be expensed over a remaining weighted-average period of 1.56 years. The Company recorded $2,622 , $4,395 and $5,901 of expense during the years ended December 31, 2015 , 2016 and 2017 , respectively, related to the Service-Based RSUs. The Company has not recorded any tax benefit related to its Service-Based RSU expense. The fair value of each Service-Based RSU granted during the year ended December 31, 2017 was estimated using the closing stock price of the Company's common stock on the date of grant. Employee Stock Purchase Plan On May 7, 2013, the Company adopted an employee stock purchase plan (the "ESPP"), pursuant to which eligible employees may purchase shares of the Company's common stock at 85% of the fair market value of the common stock on the last trading day of two consecutive, non-concurrent offering periods each year. The Company has reserved 2,500,000 shares of its common stock for issuance under the ESPP, and the first offering period under the ESPP commenced on September 1, 2013. The Company recorded $50 , $51 and $41 of expense related to the ESPP during the years ended December 31, 2015 , 2016 and 2017 , respectively. The Company has not recorded any tax benefit related to its ESPP expense. As of December 31, 2017 , the Company had issued an aggregate of 319,460 shares pursuant to the ESPP. Non-Qualified Non-Public Subsidiary Unit Options In September 2013, the Company’s subsidiary Renewables adopted a unit option plan and granted unit option awards thereunder (the “Renewables Option Awards”) to certain individuals. 150,000 Class B units representing membership interests in Renewables were initially reserved for issuance under the Renewables unit option plan. The following table summarizes activity of Renewables Option Awards: Number of Weighted Weighted Aggregate Options Outstanding as of December 31, 2014 115,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired (7,000 ) 40.80 Options Outstanding as of December 31, 2015 108,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired — — Options Outstanding as of December 31, 2016 108,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired (108,000 ) 40.80 Options Outstanding and Exercisable as of December 31, 2017 — The grant date fair value of unit options granted in September 2013 was $31.65 , which was determined contemporaneously with the unit option grants. The volatility amount used was estimated based on the historical volatility of a certain peer group of Renewables for a period commensurate with the expected life of the unit options granted. The expected life used was Renewables' anticipated exercise periods for its outstanding unit options. The risk free interest rate used was based on the U.S. Treasury yield curve for the expected life of the unit options at the time of grant. Renewables recorded $1,115 , $803 and $0 of unit option expense during the years ended December 31, 2015 , 2016 and 2017 , respectively. Renewables has not recorded any tax benefit related to its unit option expense. In connection with the closing of the BP Transaction, all holders of outstanding Renewables Option Awards entered into a surrender agreement with the Company and Renewables, pursuant to which (i) all Renewables Option Awards held by holders who were not members of Renewables’ Board of Managers were surrendered and canceled in full in exchange for, upon the closing of the BP Transaction and Renewables’ receipt of any future cash payment pursuant to the terms of the APA, a cash payment in an amount determined based on such holder’s percentage ownership of Renewables following a cashless “net exercise” of such holder’s Renewables Option Awards, and (ii) all Renewables Option Awards held by members of Renewables’ Board of Managers were surrendered and canceled in full in exchange for, upon the closing of the BP Transaction and Renewables’ receipt of any future cash payment pursuant to the terms of the APA, awards of shares of the Company's common stock (the “Company Stock Awards”). The number of shares of the Company's common stock subject to each Company Stock Award was calculated by dividing the cash payment to which the applicable holder would have been entitled as described in (i) above by the closing price of the Company's common stock on March 31, 2017, the closing date of the BP Transaction. All Company Stock Awards were granted under the 2016 Plan and are fully vested upon grant, and the shares subject to such awards are freely tradable upon issuance, subject to applicable securities laws relating to shares held by the Company's affiliates.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The components of loss before income taxes for the years ended December 31, 2015 , 2016 and 2017 are as follows: 2015 2016 2017 U.S. $ (111,437 ) $ 7,150 $ (44,535 ) Foreign (22,407 ) (19,535 ) (38,770 ) Total loss before income taxes $ (133,844 ) $ (12,385 ) $ (83,305 ) The provision for income taxes consists of the following: 2015 2016 2017 Current: Federal $ 9 $ (226 ) $ 31 State 248 93 231 Foreign 912 567 224 Total current 1,169 434 486 Deferred: Federal 337 478 (978 ) State 71 75 (184 ) Foreign 37 352 (1,238 ) Total deferred 445 905 (2,400 ) Total $ 1,614 $ 1,339 $ (1,914 ) The Company's federal and state tax benefit from the utilization of net operating loss carryovers for the year ended December 31, 2017 was $6,002 and $1,328 respectively. Income tax expense (benefit) for the years ended December 31, 2015 , 2016 and 2017 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following: 2015 2016 2017 Computed expected tax (benefit) $ (46,846 ) $ (4,335 ) $ (29,157 ) Nondeductible expenses 24,998 5,971 13,420 Tax rate differential on foreign earnings 3,701 720 11,860 Impact of federal income tax rate change — — 59,729 Tax credits (9,988 ) (9,331 ) (27 ) Other (372 ) 833 2,376 Change in valuation allowance 30,121 7,481 (60,115 ) Total tax expense $ 1,614 $ 1,339 $ (1,914 ) On December 21, 2017, the Tax Cuts and Jobs Act ("TCJA") was enacted. Among other things, the TCJA reduces the U.S. federal corporate tax rate from 35 percent to 21 percent beginning on January 1, 2018, requires companies to pay a one-time transition tax on certain previously unremitted earnings of non-U.S. subsidiaries, creates new taxes on certain foreign sourced earnings and imposes additional limitations on certain deductions, including interest expense and net operating losses arising after 2017. The Company has assessed the impact of the TCJA and is not subject to the one-time transition tax. The Company remeasured certain deferred tax assets and liabilities and uncertain tax positions based on the rates at which they are expected to reverse in the future, which is generally 21 percent under the TCJA. The decrease in the Company's net deferred tax assets was offset by a corresponding decrease in our valuation allowance. During the year ended December 31, 2015, federal tax legislation enacted AFTC through December 31, 2016 with retroactive effect to January 1, 2015. Additionally, in 2013 federal tax guidance was issued that clarified that the AFTC in excess of the Company's fuel tax obligation, which is collected from customers, can be excluded from taxable income. The AFTC, which had previously expired on December 31, 2016, was reinstated on February 9, 2018 to apply to vehicle fuel sales made from January 1, 2017 through December 31, 2017. As a result, all AFTC revenue for vehicle fuel the Company sold in the 2017 calendar year, will be recognized and collected subsequent to December 31, 2017. The Company recorded a federal tax benefit of $9,298 , $9,112 and $0 related to the exclusion of AFTC associated with 2015 , 2016 and 2017 fuel sales in excess of its fuel tax obligation, respectively. These amounts increased the Company's deferred tax asset attributed to its federal net operating loss carryforwards and the Company's deferred tax asset valuation allowance. Deferred tax assets and liabilities result from differences between the financial statement carrying amounts and the tax bases of existing assets and liabilities. The tax effect of temporary differences that give rise to deferred tax assets and liabilities as of December 31, 2016 and 2017 are as follows: 2016 2017 Deferred tax assets: Accrued expenses $ 4,566 $ 5,775 Sales-type leases 55 — Alternative minimum tax and general business credits 6,137 6,291 Stock option expense 26,154 13,782 Other 1,168 881 Loss carryforwards 181,884 103,892 Total deferred tax assets 219,964 130,621 Less valuation allowance (195,968 ) (120,834 ) Net deferred tax assets 23,996 9,787 Deferred tax liabilities: Depreciation and amortization (19,364 ) (3,600 ) Goodwill (5,599 ) (4,206 ) Investments in joint ventures and partnerships (1,432 ) (1,981 ) Total deferred tax liabilities (26,395 ) (9,787 ) Net deferred tax liabilities $ (2,399 ) $ — As of December 31, 2017 , the Company had federal, state and foreign net operating loss carryforwards of approximately $417,402 , $284,955 and $727 , respectively. The Company's federal, state and foreign net operating loss carryforwards will, if not utilized, expire beginning in 2026, 2018 and 2030, respectively. The Company also has federal tax credit carryforwards of $6,083 that will expire beginning in 2026. Due to the change of ownership provisions of Internal Revenue Code Section 382, utilization of a portion of the Company's net operating loss and tax credit carryforwards may be limited in future periods. In assessing the realizability of the net deferred tax assets, management considers whether it is more likely than not that some or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers projected future taxable income and tax planning strategies in making this assessment. As of December 31, 2016 and 2017 , the Company provided a valuation allowance of $195,968 , and $120,834 , respectively, to reduce the net deferred tax assets due to uncertainty surrounding the realizability of these assets. The increase in the valuation allowance for the year ended December 31, 2016 of $6,765 was primarily attributable to operating losses incurred in certain jurisdictions for which a full valuation allowance was established. The decrease in the valuation allowance for the year ended December 31, 2017 of $(75,134) was primarily attributable to the reduction on the federal tax rate, the CEC combination, and was offset by an increase related to the adoption of ASU 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payments Accounting , which eliminated the requirement to defer recognition of an excess tax benefit until the benefit is realized through a reduction to income taxes payable. For the year ended December 31, 2017 , the Company did not have any offshore earnings of certain non-U.S. subsidiaries which are permanently reinvested outside the United States. The Company does not recognize the impact of a tax position in its financial statements unless the position is more likely than not to be sustained, based on the technical merits of the position. The Company has unrecognized tax benefits of $34,065 at December 31, 2017 including $692 of tax benefits that, if recognized, would reduce the Company's annual effective tax rate. The remaining $33,373 , if recognized, would not result in a tax benefit since it would be fully offset with a valuation allowance. The following is a tabular reconciliation of the total amounts of unrecognized tax benefits for the years ended December 31, 2015 , 2016 and 2017 : Unrecognized tax benefit—December 31, 2015 $ 27,497 Gross increases—tax positions in current year 4,556 Gross increases—tax positions in prior years 17,549 Unrecognized tax benefit—December 31, 2016 49,602 Gross increases—tax positions in current year — Gross decreases—tax positions in prior years (15,537 ) Unrecognized tax benefit—December 31, 2017 $ 34,065 The increase in the Company's unrecognized tax benefits during the year ended December 31, 2016 is primarily attributable to the portion of AFTC revenue that was offset by the fuel tax the Company collected from its customers as an unrecognized tax benefit during the year ended December 31, 2016 . The Company believes the portion of AFTC revenue that is offset by the fuel tax the Company collects from its customers can be excluded from taxable income, although the ultimate outcome of this tax position is uncertain. The Company increased its reserve for unrecognized tax positions in the year ended December 31, 2016 . The increase in the Company's reserve for unrecognized tax positions was attributable to the write-off of unamortized debt issuance costs resulting from the Company's termination of its Credit Agreement with GE on December 15, 2015. Although the ultimate outcome of this tax position is uncertain, the Company believes that this charge can be deducted in determining its U.S. taxable income for the year ended December 31, 2015. As unrecognized tax positions are not recognized for financial reporting purposes, these positions do not have an impact on the Company's consolidated balance sheets, statement of operations, or statement of cash flows. If these positions were to be sustained, then there would be an increase in the Company's deferred tax assets attributed to its federal and state net operating loss carryforwards, as well as an increase to the amount of the Company's deferred tax asset valuation allowance. The decrease in the Company's unrecognized tax benefits during the year ended December 31, 2017 is primarily attributable to the reduction on the federal tax rate under the TCJA. FASB authoritative guidance requires the Company to accrue interest and penalties where there is an underpayment of taxes based on the Company's best estimate of the amount ultimately to be paid. The Company's policy is to recognize interest accrued related to unrecognized tax benefits and penalties as income tax expense. In addition to the unrecognized tax benefits noted above, the Company accrued $241 and $308 of interest expense as of December 31, 2016 and 2017 , respectively. The Company recognized interest expense related to uncertain tax positions of $58 , $62 and $67 for the years ended December 31, 2015 , 2016 and 2017 , respectively. The Company is under examination by the Internal Revenue Service ("IRS") for its U.S. federal income tax returns for the year ended December 31, 2015. The IRS has not proposed any significant adjustments to the Company’s tax positions for which the Company is not adequately reserved. The Company is subject to taxation in the United States and various states and foreign jurisdictions. The Company's tax years for 2013 through 2017 are subject to examination by various tax authorities. While the Company is no longer subject to U.S. examination for years before 2014 , and for state tax examinations for years before 2013 , taxing authorities can adjust the net operating losses that arose in earlier years if and when the net operating losses reduce future income. In addition, the Company is required to indemnify SAFE&CEC S.r.l. for taxes that are imposed on CEC for pre -contribution tax periods. A number of years may elapse before an uncertain tax position is finally resolved. It is often difficult to predict the final outcome or the timing of resolution of an uncertain tax position, but the Company believes that its reserves for income taxes reflect the most probable outcomes. The Company adjusts the reserve, as well as the related interest and penalties, in light of changing facts and circumstances. The amount of penalties accrued is immaterial. Settlement of any particular position would usually require the use of cash and result in the reduction of the related reserve, or there could be a change in the amount of the Company's net operating loss. The resolution of a matter would be recognized as an adjustment to the provision for income taxes at the effective tax rate in the period of resolution. The Company does not expect a significant increase or decrease in its uncertain tax positions within the next twelve months.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Environmental Matters The Company is subject to federal, state, local and foreign environmental laws and regulations. The Company does not anticipate any expenditures to comply with such laws and regulations that would have a material impact on the Company's consolidated financial position, results of operations or liquidity. The Company believes that its operations comply, in all material respects, with applicable federal, state, local and foreign environmental laws and regulations. Litigation, Claims and Contingencies The Company may become party to various legal actions that arise in the ordinary course of its business. The Company is also subject to audit by tax and other authorities for varying periods in various federal, state, local and foreign jurisdictions, and disputes may arise during the course of these audits. It is impossible to determine the ultimate liabilities that the Company may incur resulting from any of these lawsuits, claims, proceedings, audits, commitments, contingencies and related matters or the timing of these liabilities, if any. If these matters were to ultimately be resolved unfavorably, it is possible that such an outcome could have a material adverse effect upon the Company’s consolidated financial position, results of operations, or liquidity. The Company, does not, however, anticipate such an outcome and it believes the ultimate resolution of these matters will not have a material adverse effect on the Company’s consolidated financial position, results of operations, or liquidity. Operating Lease Commitments The Company leases facilities, including the land for its LNG production plant in Boron, California and certain equipment under noncancelable operating leases expiring at various dates through 2038 . The following schedule represents the Company's future minimum lease obligations under all noncancelable operating leases as of December 31, 2017 : Fiscal year: 2018 $ 5,626 2019 5,169 2020 4,342 2021 3,183 2022 2,216 Thereafter 13,836 Total future minimum lease payments $ 34,372 Rent expense, including variable rent, totaled $8,629 , $11,058 , and $7,878 for the years ended December 31, 2015 , 2016 and 2017 , respectively. Long-Term Take-or-Pay Natural Gas Purchase Contracts In October 2007, the Company entered into an LNG supply contract with Desert Gas Services (formerly known as Spectrum Energy Services, LLC) ("DGS") to purchase LNG, on a take-or-pay basis, starting in March 2010 and expiring in March 2020. For the years ended December 31, 2015 , 2016 and 2017 , the Company paid approximately $11,852 , $9,692 , and $8,092 , respectively, under this contract. As of December 31, 2017 , the fixed commitments under this contract totaled approximately $4,818 , $4,818 and $1,201 for the years ending December 31, 2018 , 2019 and 2020 respectively. During 2015, the Company entered into a CNG supply contract with Jacksonville Transit Authority ("JTA") to purchase CNG, on a take-or-pay basis, starting in January 2016 and expiring in December 2020. As of December 31, 2017 , the fixed commitments under the JTA contract totaled approximately $313 , $429 and $548 for the years ending December 31, 2018 , 2019 and 2020 , respectively. Long-Term Natural Gas Purchase Contracts In June 2017, the Company's subsidiary, NG Advantage, entered into an arrangement with one of its customers for the purchase, sale and transportation of CNG over a five -year period starting in December 2018 and expiring March 2022. The arrangement is customary and ordinary course, and provides for the payment by the customer of a nonrefundable amount of $13,360 million to reserve a specified volume of CNG transportation capacity under the arrangement. As of December 31, 2017 , this amount was paid to NG Advantage, and the fixed commitments from NG Advantage under this contract totaled approximately $2,593 , $13,157 , $13,315 , $13,317 , and $10,671 for the years ending December 31, 2018 , 2019 , 2020 , 2021 , and 2022 respectively. Purchase of Natural Gas Heavy -Duty Trucks In July 2017, the Company entered into an arrangement to purchase 146 used natural gas heavy -duty trucks from a single seller, with the intention of selling the trucks to its customers. As of December 31, 2017, this arrangement represented an outstanding commitment to purchase 124 natural gas heavy -duty trucks for $ 7,936 , which were purchased by January 31, 2018.

Capitalized Lease Obligation an

Capitalized Lease Obligation and Receivables	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Capitalized Lease Obligation and Receivables	Capitalized Lease Obligation and Receivables The Company leases equipment under capital leases with a weighted-average interest rate of 6.59% . As of December 31, 2017 , future payments under these capital leases are as follows: 2018 $ 1,374 2019 1,278 2020 1,135 2021 1,044 2022 1,027 Thereafter 2,409 Total minimum lease payments 8,267 Less amount representing interest (1,213 ) Future minimum lease payments 7,054 Less current portion (1,072 ) Capital lease obligations, less current portion $ 5,982 The value of the equipment under capital leases as of December 31, 2016 and 2017 was $10,168 and $7,934 , with related accumulated amortization of $4,073 and $846 , respectively. The Company also leases certain fueling station equipment to certain customers under sales-type leases at a weighted average effective interest rate of 13.1% . The leases are payable in varying monthly installments through 2030 . As of December 31, 2017 , future receipts under these leases are as follows: 2018 $ 254 2019 186 2020 186 2021 186 2022 186 Thereafter 1,426 Total 2,424 Less amount representing interest (1,231 ) $ 1,193

401(k) Plan

401(k) Plan	12 Months Ended
	Dec. 31, 2017
Compensation and Retirement Disclosure [Abstract]
401(k) Plan	401(k) Plan The Company has established a savings plan ("Savings Plan") which is qualified under Section 401(k) of the Internal Revenue Code. Eligible employees may elect to make contributions to the Savings Plan through salary deferrals of up to 90% of their base pay, subject to Internal Revenue Code limitations. The Company may also make discretionary contributions to the Savings Plans, subject to limitations. For the years ended December 31, 2015 , 2016 and 2017 the Company contributed approximately $304 , $1,527 , and $1,336 of matching contributions to the Savings Plan, respectively.

Reportable Segments and Geograp

Reportable Segments and Geographic Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Reportable Segments and Geographic Information	Reportable Segments and Geographic Information Disclosures are required for certain information regarding operating segments, products and services, geographic areas of operation and major customers. Segment reporting is based upon the “management approach,” which assesses, how management organizes the Company’s operating segments for which separate financial information is (1) available and (2) evaluated regularly by the Chief Operating Decision Maker ("CODM") in deciding how to allocate resources and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company operates in a single segment to sell natural gas. In making operating decisions, the CODM primarily considers consolidated financial information, accompanied by volumes delivered information. The assessment of operating results and the allocation of resources among the components of the business are made by the CODM and are based on gross margins and volumes delivered by market sector and volume type. Contracts are evaluated based on the economics of a mix of products and services for a customer. The table below presents the Company's revenue, operating loss and long-lived assets by geographic area. Several of the Company's functions, including marketing, engineering, and finance are performed at the corporate level. As a result, significant interdependence and overlap exists among the Company's geographic areas. Geographic revenue data reflect internal allocations and are therefore subject to certain assumptions and the Company’s methodology. Accordingly, revenue, operating loss, and long-lived assets shown for each geographic area may not be the amounts that would have been reported if the geographic areas were independent of one another. Revenue by geographic area is categorized based on where services are rendered and finished goods are sold. Operating loss by geographic area is categorized based on the location of the entity selling the finished goods or providing the services. Long-lived assets by geographic are categorized based on the location of the assets. 2015 2016 2017 Revenue: United States $ 330,003 $ 378,497 $ 316,756 Canada 21,818 11,502 6,846 Other 32,499 12,657 17,997 Total revenue $ 384,320 $ 402,656 $ 341,599 Operating loss: United States $ (33,067 ) $ (8,693 ) $ (96,228 ) Canada (4,980 ) (4,212 ) (9,495 ) Other (3,576 ) (4,732 ) (28,724 ) Total operating loss $ (41,623 ) $ (17,637 ) $ (134,447 ) Long-lived assets: United States $ 582,644 $ 547,279 $ 465,245 Canada 68,292 66,191 373 Other 5,693 5,646 — Total long-lived assets $ 656,629 $ 619,116 $ 465,618 The Company's goodwill and intangible assets as of December 31, 2015 , 2016 and 2017 relate to its United States operations, including the operations of CEC (until completion of the CEC Combination, see Note 3 ), Clean Energy Cryogenics and NG Advantage (see Note 9 ).

Concentrations

Concentrations	12 Months Ended
	Dec. 31, 2017
Risks and Uncertainties [Abstract]
Concentrations	Concentrations During the years ended December 31, 2015 , 2016 and 2017 , three , four and two suppliers, respectively, each accounted for 10% or more of the Company's natural gas expense related to CNG and LNG purchases. During the years ended December 31, 2015 , 2016 and 2017 , no single customer accounted for 10% or more of the Company's total revenue.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 9, 2018, the AFTC was reinstated for vehicle fuel sales made from January 1, 2017 through December 31, 2017. As a result, all AFTC revenue for vehicle fuel sold by the Company in the 2017 calendar year, will be recognized and collected subsequent to December 31, 2017. On February 28, 2018, the Company entered into a guaranty agreement in connection with the June 2017 arrangement between NG Advantage and one of its customers for the purchase, sale and transportation of CNG (see Note 14 for more information). As consideration for the guaranty agreement, NG Advantage issued to the Company 19,660 common units, which increased its controlling interest in NG Advantage from 53.3% to 53.5% . On March 1, 2018, NG Advantage entered into an equipment lease agreement for CNG trailers in the amount of $8,891 , payable monthly in 72 installments.

Schedule II_ Valuation and Qual

Schedule II: Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule II: Valuation and Qualifying Accounts	Schedule II - Valuation and Qualifying Accounts (In thousands) Allowances for Allowance for Balance as of December 31, 2014 $ 752 $ 2,850 Charges (benefit) to operations 1,514 1,142 Deductions (371 ) (2 ) Balance as of December 31, 2015 1,895 3,990 Charges (benefit) to operations 1,107 1,617 Deductions (1,939 ) (4,377 ) Balance as of December 31, 2016 1,063 1,230 Charges (benefit) to operations 395 3,344 Deductions (182 ) (30 ) Balance as of December 31, 2017 $ 1,276 $ 4,544

Summary of Significant Accoun30

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments necessary to state fairly the Company's financial position, results of operations, comprehensive loss and cash flows in accordance with accounting principles generally accepted in the United States of America ("US GAAP"). All intercompany accounts and transactions have been eliminated in consolidation.
Reclassifications	Reclassifications Prior period Gain from change in fair value of derivative warrants of $1,414 and $22 for the years ended December 31, 2015 and 2016, respectively, were reclassified from a separate line item into Selling, general and administrative in the consolidated statements of operations, and the same amounts were reclassified into Accrued expenses and other in the consolidated statements of cash flows to conform to the classifications used to prepare the consolidated financial statements for the year ended December 31, 2017 . In addition, Deferred revenue of $4,134 and $3,479 for the years ended December 31, 2015 and 2016, respectively were reclassified from Accrued expenses and other as a separate line item in the consolidated statements of cash flows to conform to current period presentation. These reclassifications had no material impact on the Company’s financial position, results of operations or cash flows as previously reported. Prior period Interest income of $843 for the year ended December 31, 2015 was reclassified from Interest expense, net to a separate line item, to conform to the classifications used to prepare the consolidated financial statements for the years ended December 31, 2016 and 2017 . This reclassification had no material impact on the Company’s financial position, results of operations or cash flows as previously reported.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and may result in material effects on the Company's operating results and financial position. Significant estimates made in preparing the consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived intangible asset valuations and impairment assessments, income tax valuations, fair value measurements and stock-based compensation expense.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of three months or less on the date of acquisition to be cash equivalents.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying values of the Company's financial instruments, including cash and cash equivalents, restricted cash, short-term investments, accounts receivables, other receivables, notes receivable, accounts payable, accrued expenses and other current liabilities, capital lease obligations and notes payable, approximate their respective fair values.
Inventory	Inventory Inventory consists of raw materials and spare parts, work in process and finished goods and is stated at the lower of cost (first-in, first-out) or net realizable value. The Company writes down the carrying value of its inventory to net realizable value for estimated obsolescence or unmarketable inventory in an amount equal to the difference between the cost of inventory and its estimated realizable value based upon assumptions about future demand and market conditions, among other factors.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost. Depreciation and amortization are recognized over the estimated useful lives of the assets using the straight-line method. The estimated useful lives of depreciable assets are three to twenty years for LNG liquefaction plant assets, up to 10 years for station equipment and LNG trailers, and three to seven years for all other depreciable assets. Leasehold improvements are amortized over the shorter of their estimated useful lives or related lease terms. Periodically, the Company receives grant funding to assist in the financing of natural gas fueling station construction. The Company records the grant proceeds as a reduction of the cost of the respective asset.
Long-Lived Assets	Long-Lived Assets The Company reviews the carrying value of its long-lived assets, including property and equipment and intangible assets with finite useful lives, for impairment whenever events or changes in circumstances indicate that the carrying value of an asset or asset group may not be recoverable. Events that could result in an impairment review include, among others, a significant decrease in the operating performance of a long-lived asset or asset group or the decision to close a fueling station. Impairment testing involves a comparison of the sum of the undiscounted future cash flows of the asset or asset group to its carrying amount. If the sum of the undiscounted future cash flows exceeds the carrying amount, then no impairment exists. If the carrying amount exceeds the sum of the undiscounted future cash flows, then a second step is performed to determine the amount of impairment, if any, to be recognized. An impairment loss is recognized to the extent that the carrying amount of the asset or asset group exceeds its fair value. The fair value of the asset or asset group is based on estimated discounted future cash flows of the asset or asset group using a discount rate commensurate with the related risk. The estimate of future cash flows requires management to make assumptions and to apply judgment, including forecasting future sales and expenses and estimating useful lives of the assets. These estimates can be affected by a number of factors, including, among others, future results, demand, and economic conditions, many of which can be difficult to predict.
Finite-Lived Intangible Assets Amortization	Intangible assets with finite useful lives are amortized over their respective estimated useful lives using the straight-line method. The estimated useful lives of intangible assets with finite useful lives are from two to 20 years for technology, one to eight years for customer relationships, one to 10 years for acquired contracts, two to 10 years for trademarks and trade names, and three years for non-compete agreements.
Goodwill	Goodwill Goodwill represents the excess of costs incurred over the fair value of the net assets of acquired businesses. The Company assesses its goodwill using either a qualitative or quantitative approach to determine whether it is more likely than not that the fair value of its reporting unit is less than its carrying value. The Company is required to use judgment when applying the goodwill impairment test, including, among other considerations, the identification of reporting unit(s), the assessment of qualitative factors, and the estimation of fair value of a reporting unit in the quantitative approach. The Company determined that it is a single reporting unit for the purpose of goodwill impairment tests. The Company performs the impairment test annually on October 1, or more frequently if facts and circumstances warrant a review. The qualitative goodwill assessment includes the potential impact on a reporting unit's fair value of certain events and circumstances, including its enterprise value, macroeconomic conditions, industry and market considerations, cost factors, and other relevant entity-specific events. If it is determined, based upon the qualitative assessment, that it is more likely than not that the reporting unit's fair value is less than its carrying amount, then a quantitative impairment test is performed. The quantitative assessment estimates the reporting unit's fair value based on its enterprise value plus an assumed control premium as evidence of fair value. The estimates used to determine the fair value of the reporting unit may change based on results of operations, macroeconomic conditions, stock price fluctuations, or other factors. Changes in these estimates could materially affect our assessment of the fair value and goodwill impairment for the reporting unit.
Revenue Recognition	Revenue Recognition The Company recognizes revenue from various sources. The Company’s volume related revenue primarily consists of CNG, LNG and RNG fuel sales, RINs and LCFS Credits sales and O&M services. This revenue is recognized when the following four criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery has occurred and title and the risks and rewards of ownership have been transferred to the customer or services have been rendered; (iii) the price is fixed or determinable; and (iv) collectability is reasonably assured. Applying these factors, the Company typically recognizes revenue from the sale of natural gas fuel at the time the fuel is dispensed or, in the case of LNG sales agreements, delivered to the customers' storage facilities. The Company recognizes revenue from O&M service agreements as the related services are provided. The Company generates RIN Credits when it sells RNG as a vehicle fuel in the United States, and it generates LCFS Credits when it sells RNG and conventional natural gas for use as a vehicle fuel in California and Oregon. The Company can sell these credits to third parties who need the credits to comply with federal and state requirements. RIN and LCFS Credits are included in volume related revenues. The Company recognizes revenue from the generation of these credits when it has an agreement in place to sell the credits at a fixed or determinable price. Compressor Sales Before completion of the CEC Combination (see Note 3 ), the Company recognized compression revenue through its former subsidiary CEC when it sold non-lubricated natural gas fueling compressors and other equipment. CEC used the percentage-of-completion method of accounting to recognize revenue because its projects were small and it was able to demonstrate that it could reasonably estimate costs to complete. In these circumstances, revenue was recognized based on costs incurred in relation to total estimated costs to be incurred for a project. Station Construction Sales Beginning January 1, 2016, the Company began using the percentage of completion method to recognize revenue for station construction projects using the cost-to-cost method. Under this method, the Company estimates the percentage of completion of a project based on the costs incurred to date for the associated contract in comparison to the estimated total costs for such contract at completion. Historically, the Company recognized revenue on station construction projects using the completed contract method because the Company did not have a reliable means to make estimates of the percentage of the contract completed. Under the completed contract method, the construction projects were considered substantially complete at the earlier of customer acceptance of the fueling station or the time when fuel dispensing activities at the station began. The sale of compressors and related equipment continues to be recognized under the percentage of completion method as in previous periods. Effective January 1, 2016, the Company implemented a cost tracking system that provides for a detailed tracking of costs incurred on its station construction projects on a project by project basis. The Company also changed related accounting activities and processes to timely identify and monitor costs. As a result of this implementation, the Company is able to make reliable estimates as to the percentage of a project that is complete at the end of each reporting period. Station construction contracts are generally short-term, except for certain larger and more complex stations, which can take up to 24 months to complete. Management evaluates the performance of contracts on an individual contract basis. Contract price and cost estimates are reviewed periodically as work progresses and adjustments proportionate to the percentage of completion are reflected in contract revenues in the reporting period when such estimates are revised. The nature of accounting for contracts is such that refinements of estimates to account for changing conditions and new developments are continuous and characteristic of the process. Many factors that can affect contract profitability may change during the performance period of a contract, including differing site conditions, the availability of skilled contract labor, the performance of major suppliers and subcontractors, and unexpected changes in material costs. Changes to these factors may result in revisions to costs and income, which are recognized in the period in which the revisions become known. Provisions for estimated losses on uncompleted contracts are made in the period in which such losses become known. During the year ended December 31, 2017 , there were no significant losses on open contracts. The Company considers unapproved change orders to be contract variations for which the customer has approved the change of scope but an agreement has not been reached as to an associated price change. Change orders that are unapproved as to both price and scope are evaluated as claims. Claims have historically been insignificant. There were no significant unapproved change orders, claims, contract penalties, settlements or changes in contract estimates during the year ended December 31, 2017 . In certain transactions with its customers, the Company agrees to provide multiple products or services, including construction of and sale of a station, providing O&M services to the station, and sale of fuel to the customer. The Company evaluates the separability of revenues based on Financial Accounting Standards Board ("FASB") authoritative guidance, which provides a framework for establishing whether or not a particular arrangement with a customer has one or more revenue elements, and allows the Company to use a combination of internal and external objective and reliable evidence to develop management's best estimate of the fair value of the contract elements. If the arrangement contains a lease, the Company uses the existing evidence of fair value to separate the lease from the other elements in the arrangement. The arrangement's consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the estimated relative selling price of each deliverable, which is determined based on the historical data derived from the Company's stand-alone projects. The revenue allocated to the construction of the station is recognized using the percentage of completion method. The revenue allocated to the O&M services is recognized ratably over the term of the arrangement and sale of fuel is recognized as the fuel is delivered. See the discussion under “Alternative Fuels Excise Tax Credit” below for more information. Other The Company collects and remits taxes assessed by various governmental authorities that are imposed on and concurrent with revenue-producing transactions between the Company and our customers. These taxes may include, but are not limited to, fuel, sales and value-added taxes. The Company reports the collection of these taxes on a net basis.
Alternative Fuels Excise Tax Credit	Alternative Fuels Excise Tax Credit Under separate pieces of U.S. federal legislation, the Company has been eligible to receive a federal alternative fuels tax credit (“AFTC,” formerly known as VETC) for its natural gas vehicle fuel sales made between October 1, 2006 and December 31, 2017. The AFTC, which had previously expired on December 31, 2016, was reinstated on February 9, 2018 to apply to vehicle fuel sales made from January 1, 2017 through December 31, 2017. The AFTC credit is equal to $0.50 per gasoline gallon equivalent of CNG that the Company sold as vehicle fuel, $0.50 per liquid gallon of LNG that the Company sold as vehicle fuel through 2015, and $0.50 per diesel gallon of LNG that the Company sold as vehicle fuel in 2016 and 2017. Based on the service relationship with its customers, either the Company or its customers claims the credit. The Company records its AFTC credits, if any, as revenue in its consolidated statements of operations because the credits are fully payable and do not need to offset income tax liabilities to be received.
LNG Transportation Costs	LNG Transportation Costs The Company records the costs incurred to transport LNG to its customers in the line item Product cost of sales in the accompanying consolidated statements of operations.
Advertising Costs	Advertising Costs Advertising costs are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company recognizes compensation expense for all stock‑based payment arrangements over the requisite service period of the award. For stock options, the Company determines the grant date fair value using the Black‑Scholes option pricing model, which requires the input of certain assumptions, including the expected life of the stock‑based payment awards, stock price volatility and risk‑free interest rates. For restricted stock units, the Company determines the grant date fair value based on the closing market price of its common stock on the date of grant.
Income Taxes	Income Taxes Income taxes are computed using the asset and liability method. Under this method, deferred income taxes are recognized by applying enacted statutory tax rates applicable to future years to differences between the tax bases and financial carrying amounts of existing assets and liabilities. The impact on deferred taxes of changes in tax rates and laws, if any, are applied to the years during which temporary differences are expected to be settled and are reflected in the consolidated financial statements in the period of enactment. Valuation allowances are established when management determines it is more likely than not that deferred tax assets will not be realized. When evaluating the need for a valuation analysis, we use estimates involving a high degree of judgment including projected future US GAAP income and the amounts and estimated timing of the reversal of any deferred tax assets and liabilities. The Company has a recognition threshold and a measurement attribute for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities based on the technical merits of the position. The amount recognized is measured as the largest amount of benefit that has a greater than 50 percent likelihood of being realized upon ultimate settlement. The Company recognizes potential accrued interest and penalties related to unrecognized tax benefit in income tax expense. The Company operates within multiple domestic and foreign taxing jurisdictions and is subject to audit in these jurisdictions. These audits can involve complex issues, which may require an extended period of time to resolve. Although the Company believes that adequate consideration has been given to these issues, it is possible that the ultimate resolution of these issues could be significantly different than originally estimated.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period. Diluted net loss per share is computed by dividing the net loss attributable to Clean Energy Fuels Corp. by the weighted-average number of common shares outstanding and common shares issuable for little or no cash consideration during the period and potentially dilutive securities outstanding during the period, and therefore reflects the dilution from common shares that may be issued upon exercise or conversion of these potentially dilutive securities, such as stock options, warrants, convertible notes and restricted stock units. The dilutive effect of stock awards and warrants is computed under the treasury stock method. The dilutive effect of convertible notes and restricted stock units is computed under the if-converted method. Potentially dilutive securities are excluded from the computations of diluted net loss per share if their effect would be antidilutive.
Foreign Currency Translation	Foreign Currency Translation The Company uses the local currency as the functional currency of its foreign subsidiaries. Accordingly, all assets and liabilities outside the United States are translated into U.S. dollars at the rate of exchange in effect at the balance sheet date. Revenue and expense items are translated at the weighted-average exchange rates prevailing during the period. Foreign currency translation adjustments are recorded as accumulated other comprehensive loss in stockholders' equity.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in equity (net assets) of a business enterprise during the period from transactions and other events and circumstances from non-owner sources.
Concentration of Credit Risk	Concentration of Credit Risk Credit is extended to all customers based on financial condition, and collateral is generally not required. Concentrations of credit risk with respect to trade receivables are limited because of the large number of customers comprising the Company's customer base and dispersion across many different industries and geographies. Certain international customers, however, have historically been slower to pay on trade receivables. Accordingly, the Company continually monitors collections and payments from its customers and maintains a provision for estimated credit losses based upon its historical experience and any specific customer collection issues that it has identified. Although credit losses have historically been within the Company's expectations and the provisions established, the Company cannot guarantee that it will continue to experience the same credit loss rates that it has in the past. The Company places its cash and cash equivalents with high credit quality financial institutions. At times, such investments may be in excess of the Federal Deposit Insurance Corporation ("FDIC"), and other foreign insurance limits. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash deposits.
Recently Adopted Accounting Changes and Recently Issued Accounting Standards	Recently Adopted Accounting Changes and Recently Issued Accounting Standards Recently Adopted Accounting Changes In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation (Topic 718): Scope Modification Accounting . This ASU allows entities to make certain changes to awards without accounting for them as modifications. It does not change the accounting for modifications. Specifically, an entity will not apply modification accounting if all of the following are the same immediately before and after the change: (1) the award's fair value (or calculated value or intrinsic value if those measurement methods are used), (2) the award's vesting conditions, and (3) the award's classification as an equity or liability instrument. The new standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. Early adoption is permitted for interim or annual periods in which the financial statements have not been issued. The Company elected to early adopt this ASU during the year ended December 31, 2017, which did not have any impact on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard eliminates the requirement to determine the fair value of individual assets and liabilities of a reporting unit to measure goodwill impairment. Under the amendments in the new ASU, goodwill impairment testing will be performed by comparing the fair value of the reporting unit with its carrying amount and recognizing an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value. The new standard is effective for annual and interim goodwill impairment tests in fiscal years beginning after December 15, 2019, which for the Company is the first quarter of 2020 and should be applied on a prospective basis. Early adoption is permitted for annual or interim goodwill impairment testing performed after January 1, 2017. The Company elected to early adopt this ASU during the year ended December 31, 2017, which did not have any impact on its consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The new standard provides guidance to entities to assist with evaluating when a set of transferred assets and activities (collectively, the "set") is a business and provides a criteria to determine when a set is not a business. Under the new guidance, when substantially all of the fair value of gross assets acquired (or disposed of) is concentrated in a single identifiable asset, or group of similar assets, the assets acquired would not represent a business. Also, to be considered a business, an acquisition would have to include an input and a substantive process that together significantly contribute to the ability to produce outputs. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017, which for the Company is the first quarter of 2018 and should be applied on a prospective basis to any transactions occurring within the period of adoption. Early adoption is permitted for interim or annual periods in which the financial statements have not been issued. The Company elected to early adopt this ASU during the year ended December 31, 2017, which did not have any impact on its consolidated financial statements and related disclosures. In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. Under the new standard, the selling (transferring) entity is required to recognize a current tax expense or benefit upon transfer of the asset. Similarly, the purchasing (receiving) entity is required to recognize a deferred tax asset or liability, as well as the related deferred tax benefit or expense, upon purchase or receipt of the asset. The Company early adopted the standard as of January 1, 2017. This election was implemented under the modified retrospective approach, resulting in a $303 increase in accumulated deficit representing the cumulative recognition of the income tax consequences of intra-entity transfers of assets other than inventory that occurred before the adoption date. In March 2016, the FASB issued ASU 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payments Accounting . The new standard was issued to simplify the accounting for share-based payment transactions, including income tax consequences, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. The Company adopted the standard as of January 1, 2017 and in connection with the adoption, the Company elected to recognize forfeitures when they occur. Previously, the Company estimated a forfeiture rate in accordance with prior guidance. This election was implemented under the modified retrospective approach with a cumulative effect of an increase in accumulated deficit of $194 , net of tax. This adjustment represents the cumulative additional compensation expense that would have been amortized through the date of adoption had this accounting policy election been in place. This ASU also eliminates the requirement to defer recognition of an excess tax benefit until the benefit is realized through a reduction to taxes payable. As a result, the Company’s deferred tax asset relating to its net operating loss carryovers increased by $8,600 with a corresponding increase in the deferred tax asset valuation allowance. The Company also adopted, on a prospective basis, (1) the classification of excess tax benefits as an operating activity in the consolidated statements of cash flows and (2) the exclusion of the amount of excess tax benefits when applying the treasury stock method for the Company’s diluted loss per share calculation. Neither of these adoptions had a material impact on the Company's cash flows or diluted loss per share calculation for the year ended December 31, 2017. Additionally, the Company continues to (1) classify cash paid by the Company for directly withholding shares for tax withholding purposes as a financing activity in the consolidated statements of cash flows and (2) withhold the statutory minimum taxes for participants in the Company’s stock-based compensation plans. Recently Issued Accounting Standards In January 2017, the FASB issued ASU 2017-03, Accounting Changes and Error Corrections (Topic 250) and Investments - Equity Method and Joint Ventures (Topic 323): Amendments to SEC Paragraphs Pursuant to Staff Announcements at the September 22, 2016 and November 17, 2016 EITF Meetings . This ASU requires registrants to disclose the effect that ASU 2014-09 ( Revenue from Contracts with Customers (Topic 606 ), ASU 2016-02 ( Leases (Topic 842 ), and ASU 2016-13 ( Financial Instruments--Credit Losses ( Topic 326 ) will have on their financial statements when adopted in a future period. In cases where a registrant cannot reasonably estimate the impact of the adoption, additional qualitative disclosures should be considered to assist the reader in assessing the significance of the standard’s impact on its financial statements. This guidance was effective upon issuance and other than enhancements to the qualitative disclosures regarding the future adoption of the referenced ASUs above, adoption of this ASU is not expected to have any impact on the Company’s consolidated financial statements and related disclosures. In December 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash. The new standard requires restricted cash and restricted cash equivalents to be included as components of total cash and cash equivalents as presented on the statement of cash flows. This pronouncement is effective for reporting periods beginning after December 15, 2017, which for the Company is the first quarter of 2018. Early adoption is allowed in an interim or annual reporting period. The Company is evaluating this ASU, but does not expect it to have a material impact on its consolidated financial statements and related disclosures. In September 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Payments . The new standard provides clarification as to the classification of certain transactions as operating, investing or financing activities. This pronouncement is effective for reporting periods beginning after December 15, 2017, with early adoption permitted. The Company is evaluating this ASU which is not expected to have a material impact on its consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . The new standard amends the impairment model to utilize an expected loss methodology in place of the currently used incurred loss methodology, which will result in the more timely recognition of losses. This pronouncement is effective for reporting periods beginning after December 15, 2019, which for the Company is the first quarter of 2020. The Company will evaluate the impact this ASU will have on its consolidated financial statements and related disclosures. In February 2016, the FASB issued ASU 2016-02, Leases . The new standard requires most leases to be recognized on the balance sheet which will increase reported assets and liabilities. Lessor accounting remains substantially similar to current guidance. The new standard is effective for annual and interim periods in fiscal years beginning after December 15, 2018, which for the Company is the first quarter of 2019, and mandates a modified retrospective transition method. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures. In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities . The new standard requires equity investments to be measured at fair value with changes in fair value recognized in net income, simplifies the impairment assessment of equity investments without readily determinable fair values, eliminates the requirement to disclose the methods and significant assumptions used to estimate fair value, requires use of the exit price notion when measuring fair value, requires separate presentation in certain financial statements, and requires an evaluation of the need for a valuation allowance on a deferred tax asset related to available-for-sale securities. The new standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. The Company is evaluating this ASU which is not expected to have a material impact on its consolidated financial statements and related disclosures. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) , which amends the guidance in former ASC Topic 605, Revenue Recognition , to provide a single, comprehensive revenue recognition model for all contracts with customers. The new standard requires an entity to recognize revenue in a manner that depicts the transfer of promised goods or services to customers in amounts that reflect the consideration to which an entity expects to be entitled in exchange for those goods or services. The new standard also requires entities to enhance disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. The standard is effective for fiscal years beginning after December 15, 2017, which for the Company is the first quarter of 2018. The standard may be applied retrospectively to all prior periods presented (“full retrospective method”), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (“modified retrospective method”). The Company intends to adopt the new standard using the modified retrospective method. Substantially all of the Company’s current revenue is volume related which primarily consists of natural gas fuel sales and O&M services. The remaining revenue is related to station construction and AFTC, RINS, and LCFS Credits sales. While the assessment is ongoing and not complete, the Company does not expect any material changes to the timing or amount of revenue under the new standard. As the assessment nears completion, the Company has also identified and is implementing any necessary changes to its processes, internal controls and systems configurations from the implementation of the new revenue standard. Additionally, the Company’s revenue disclosures are expected to expand and may require judgment in certain areas. The Company does not expect any significant changes to other accounting policies from the adoption of the new revenue standard.

Summary of Significant Accoun31

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of inventories	Inventories consisted of the following as of December 31, 2016 and 2017 : 2016 2017 Raw materials and spare parts (1) $ 24,843 $ 35,145 Work in process 845 — Finished goods 3,856 93 Total inventory $ 29,544 $ 35,238 (1) During the year ended December 31, 2017, $19,394 in station parts were reclassified from construction in progress within Land, property, and equipment, net, into Inventory in the consolidated balance sheets because they will primarily be used for stations to be sold. See Note 2 for more information.
Schedule of finite-lived intangible assets	The Company's intangible assets as of December 31, 2016 and 2017 were as follows: 2016 2017 Technology $ 54,400 $ — Customer relationships 16,576 5,376 Acquired contracts 4,384 4,384 Trademark and trade names 8,200 2,700 Non-compete agreements 2,060 860 Total intangible assets 85,620 13,320 Less accumulated amortization (37,591 ) (9,730 ) Foreign currency rate change (9,329 ) — Net intangible assets $ 38,700 $ 3,590
Schedule of goodwill	The following table summarizes the activity related to the carrying amount of goodwill: Balance as of December 31, 2015 $ 91,967 Foreign currency translation adjustment 1,051 Balance as of December 31, 2016 93,018 Goodwill reduced during the year (1) (30,154 ) Foreign currency translation adjustment 1,464 Balance as of December 31, 2017 $ 64,328 (1) The Company reduced its goodwill balance by $26,576 when it sold certain assets of its subsidiary, Clean Energy Renewable Fuels ("Renewables"), on March 31, 2017, and by $3,578 when it contributed CEC to SAFE&CEC S.r.l. on December 29, 2017 (all described in Note 3 ).
Summary of revenue by group of similar products	The table below and the following discussion describe the Company’s revenue by group of similar products, services or other revenue sources. Year Ended December 31, (in thousands) 2015 2016 2017 Volume -Related $ 260,629 $ 283,814 $ 264,880 Compressor Sales 54,497 27,262 23,527 Station Construction Sales 37,830 64,942 51,854 AFTC (1) 30,986 26,638 — Other 378 — 1,338 $ 384,320 $ 402,656 $ 341,599 (1) Formerly known as VETC.
Schedule of potentially dilutive securities that have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive	The following potentially dilutive securities have been excluded from the diluted net loss per share calculations because their effect would have been antidilutive: 2015 2016 2017 Stock options 11,487,938 11,467,796 8,613,854 Warrants 2,130,682 — — Convertibles notes 35,185,979 16,573,799 14,991,521 Restricted stock units 3,419,776 2,072,304 1,832,575

Asset Impairments, Other Char32

Asset Impairments, Other Charges, and Inventory Valuation Provision (Tables)	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Restructuring and impairment charges	The following table summarizes these charges: Year Ended December 31, 2017 Workforce reduction and related charges $ 3,057 CEC asset impairments 32,274 Station closures and related charges 25,557 LCFS Credits charge 7,046 Total asset impairments and other charges $ 67,934 Inventory valuation provision 13,158 Total charges $ 81,092 The following table summarizes the charges related to the foregoing that will be settled with cash payments and their related liability balances as of December 31, 2017 : Charges Cash Payments Made in the Year Ended December 31, 2017 Balance as of December 31, 2017 Employee severance $ 2,757 (2,757 ) $ — Lease termination fees and AROs for station closures 4,083 (70 ) 4,013 $ 6,840 (2,827 ) $ 4,013

Restricted Cash (Tables)

Restricted Cash (Tables)	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Schedule of components of restricted cash	Short-term restricted cash as of December 31, 2016 and 2017 consisted of the following: December 31, 2016 December 31, 2017 Short-term restricted cash: Standby letters of credit $ 1,753 $ 1,127 Canton Bonds (see Note 11) 3,665 — Held in escrow 1,578 — Total short-term restricted cash $ 6,996 $ 1,127

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of short-term investments	Short-term investments as of December 31, 2016 consisted of the following: Amortized Gross Estimated Municipal bonds and notes $ 8,791 $ (4 ) $ 8,787 Corporate bonds 21,517 (7 ) 21,510 Certificate of deposits 43,421 — 43,421 Total short-term investments $ 73,729 $ (11 ) $ 73,718 Short-term investments as of December 31, 2017 consisted of the following: Amortized Gross Estimated Municipal bonds and notes $ 21,414 $ (49 ) $ 21,365 Zero coupon bonds 54,159 (33 ) 54,126 Corporate bonds 55,109 (40 ) 55,069 Certificate of deposits 10,902 — 10,902 Total short-term investments $ 141,584 $ (122 ) $ 141,462

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of information by level for assets and liabilities that are measured at fair value on a recurring basis	The following tables provide information by level for assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2016 and December 31, 2017 , respectively: Description Balance at December 31, 2016 Level 1 Level 2 Level 3 Assets: Available-for-sale securities(1): Municipal bonds and notes $ 8,787 $ — $ 8,787 $ — Corporate bonds 21,510 — 21,510 — Certificate of deposits 43,421 — 43,421 — Liabilities: Warrants(2) 581 — — 581 Description Balance at December 31, 2017 Level 1 Level 2 Level 3 Assets: Available-for-sale securities(1): Municipal bonds and notes $ 21,365 $ 21,365 $ — Zero coupon bonds 54,126 54,126 — Corporate bonds 55,069 55,069 — Certificate of deposits 10,902 10,902 — Liabilities: Warrants(2) 536 — — 536 _______________________________________________________________________________ (1) Included in short-term investments in the consolidated balance sheets. See Note 5 for more information. (2) Included in accrued liabilities and other long-term liabilities in the consolidated balance sheets.
Reconciliation of the beginning and ending balances of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)	The following table provides a reconciliation of the beginning and ending balances of items measured at fair value on a recurring basis as shown in the tables above that used significant unobservable inputs (Level 3): Liabilities: Warrants 2016 2017 Beginning Balance $ 632 $ 581 Gain included in earnings (51 ) (45 ) Ending Balance $ 581 $ 536

Other Receivables (Tables)

Other Receivables (Tables)	12 Months Ended
	Dec. 31, 2017
Receivables [Abstract]
Schedule of other receivables	Other receivables as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Loans to customers to finance vehicle purchases $ 7,416 $ 4,746 Accrued customer billings 4,308 10,072 Fuel tax credits 6,358 177 Other 3,852 4,240 Total other receivables $ 21,934 $ 19,235

Land, Property and Equipment (T

Land, Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Summary of land, property and equipment	Land, property and equipment as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Land $ 2,858 $ 2,858 LNG liquefaction plants 94,634 94,634 RNG plants (1) 47,545 — Station equipment (2) 341,605 304,090 Trailers 54,985 70,906 Other equipment (2) 93,118 88,313 Construction in progress (2) (3) 117,662 74,905 752,407 635,706 Less accumulated depreciation (268,484 ) (268,401 ) Total land, property and equipment, net $ 483,923 $ 367,305 (1) The RNG plants were sold in the BP Transaction (see Note 3 for more information). (2) Certain of these assets were written down during the year ended December 31, 2017 (see Note 2 for more information). (3) During the year ended December 31, 2017 , $19,394 in station parts were reclassified from construction in progress within Land, property, and equipment, net, into Inventory in the consolidated balance sheets because they will primarily be used for stations to be sold (see Note 2 for more information).

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Schedule of accrued liabilities	Accrued liabilities as of December 31, 2016 and 2017 consisted of the following: 2016 2017 Accrued alternative fuels incentives (1) 9,840 2,954 Accrued employee benefits 4,317 2,378 Accrued interest 1,849 1,486 Accrued gas and equipment purchases 11,657 8,722 Accrued property and other taxes 4,572 4,582 Salaries and wages 12,293 8,363 Other (2) 8,073 13,783 Total accrued liabilities $ 52,601 $ 42,268 (1) Includes the amount of RINs and LCFS Credits and, as of December 31, 2016, the amount of AFTC payable to third parties. No AFTC amounts were accrued as of December 31, 2017 because, as of that date, the AFTC had expired (subsequent to December 31, 2017, however, the AFTC was reinstated for vehicle fuel sales made from January 1, 2017 through December 31, 2017). See Notes 1 and 19 for more information about AFTC. (2) The amount as of December 31, 2017 includes lease termination fees and AROs related to closure of certain fueling stations (see Note 2 for more information) and working capital adjustments, funding for certain commitments, and transaction fees incurred as a result of the CEC Combination (see Note 3 for more information).

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of long-term debt	Debt and capital lease obligations as of December 31, 2016 and 2017 consisted of the following and are further discussed below: December 31, 2016 Principal Balances Unamortized Debt Financing Costs Balance, Net of Financing Costs 7.5% Notes (1) $ 150,000 274 $ 149,726 5.25% Notes 110,450 1,088 $ 109,362 PlainsCapital Bank 23,500 — $ 23,500 Canton Bonds 9,520 373 $ 9,147 Capital lease obligations 6,028 — $ 6,028 Other debt 14,850 237 $ 14,613 Total debt and capital lease obligations 314,348 1,972 312,376 Less amounts due within one year (6,126 ) (183 ) (5,943 ) Total long-term debt and capital lease obligations $ 308,222 $ 1,789 $ 306,433 December 31, 2017 Principal Balances Unamortized Debt Financing Costs Balance, Net of Financing Costs 7.5% Notes (1) 125,000 131 $ 124,869 5.25% Notes 110,450 454 $ 109,996 NG Advantage debt 17,185 259 $ 16,926 Capital lease obligations 7,054 — $ 7,054 Other debt 1,242 — $ 1,242 Total debt and capital lease obligations 260,931 844 260,087 Less amounts due within one year (140,223 ) (524 ) (139,699 ) Total long-term debt and capital lease obligations $ 120,708 $ 320 $ 120,388 (1) Includes $65,000 and $0 in principal amount held by Mr. Pickens as of December 31, 2016 and December 31, 2017 , respectively, which is classified as “Long-term debt, related party” on the consolidated balance sheets. See the description below for more information
Summary of aggregate maturities of long term debt and capital lease obligations	The following is a summary of the aggregate maturities of debt and capital lease obligations for each of the yearly periods subsequent to December 31, 2017 : 2018 2019 2020 2021 2022 Thereafter 7.5% Notes (1) 25,000 50,000 50,000 — — — 5.25% Notes 110,450 — — — — — NG Advantage debt 3,483 3,242 3,413 3,055 2,665 1,327 Capital lease obligations 1,072 1,021 925 871 917 2,248 Other debt 218 229 240 247 215 93 Total $ 140,223 $ 54,492 $ 54,578 $ 4,173 $ 3,797 $ 3,668

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Summary of At-The-Market program activity	The following table summarizes the activity under the ATM Program for the periods presented: Year ended December 31, Year ended December 31, Year ended December 31, (in 000s, except per-share amounts) 2015 2016 2017 Gross proceeds $ 6,943 $ 103,591 $ 10,767 Fees and issuance costs 493 2,612 311 Net proceeds $ 6,450 $ 100,979 $ 10,456 Shares issued 1,561,902 31,064,434 3,802,500
Summary of compensation expense and related income tax benefit related to the stock-based compensation expense recognized	The following table summarizes the compensation expense and related income tax benefit related to the Company's stock-based compensation arrangements recognized in the consolidated statements of operations during the periods presented: Years Ended December 31, 2015 2016 2017 Stock-based compensation expense, net of $0 tax in 2015, 2016 and 2017 (1) $ 10,779 $ 8,092 $ 8,423 (1) $300 of stock-based compensation expense for the year ended December 31, 2017 is recorded in asset impairments and other charges in the consolidated statements of operations and is reported in non-cash portion of asset impairments and other charges in the consolidated statements of cash flows. See Note 2 for more information.
Summary of stock option activity	The following table summarizes the Company's stock option activity: Number of Weighted Weighted Aggregate Options Outstanding as of December 31, 2014 11,486,301 $ 11.91 Granted 1,415,200 5.39 Exercised (608,279 ) 2.96 Forfeited or Expired (805,284 ) 14.04 Options Outstanding as of December 31, 2015 11,487,938 $ 11.44 Granted 284,750 3.63 Exercised — — Forfeited or Expired (304,892 ) 11.30 Options Outstanding as of December 31, 2016 11,467,796 $ 11.25 Granted 1,139,500 2.83 Exercised — — Forfeited or Expired (3,993,442 ) 12.34 Options Outstanding as of December 31, 2017 8,613,854 $ 9.62 4.65 — Options Exercisable as of December 31, 2017 7,118,956 $ 10.91 3.81 — Options Vested and Expected to Vest as of December 31, 2017 8,613,854 $ 9.62 4.65 — The following table summarizes activity of Renewables Option Awards: Number of Weighted Weighted Aggregate Options Outstanding as of December 31, 2014 115,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired (7,000 ) 40.80 Options Outstanding as of December 31, 2015 108,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired — — Options Outstanding as of December 31, 2016 108,000 $ 40.80 Options granted — — Options exercised — — Options forfeited or expired (108,000 ) 40.80 Options Outstanding and Exercisable as of December 31, 2017 —
Schedule of assumptions used to estimate the fair value of each award using the Black-Scholes option pricing model	The fair value of each stock option granted was estimated as of the date of grant using the Black-Scholes option pricing model and using the following assumptions: Years Ended December 31, 2015 2016 2017 Dividend yield 0.0% 0.0% 0.0% Expected volatility 59.2% to 72.0% 61.1% to 70.8% 63.61% Risk-free interest rate 1.7% to 1.8% 1.2% to 2.0% 2.05% Expected life in years 6.0 6.0 6.0
Summary of the Company's RSU activity	The following table summarizes the Company's Service-Based RSU activity: Number of Weighted Weighted RSU Outstanding as of December 31, 2014 822,752 $ 5.82 Granted 1,167,750 5.38 Vested (283,726 ) 5.94 Forfeited or expired (56,000 ) 5.57 RSU Outstanding as of December 31, 2015 1,650,776 $ 5.50 Granted 850,125 3.63 Vested (726,687 ) 5.53 Forfeited or expired (130,910 ) 4.91 RSU Outstanding as of December 31, 2016 1,643,304 $ 4.56 Granted 2,835,331 1.36 Vested (2,840,584 ) 1.97 Forfeited or expired (139,976 ) 4.69 RSU Outstanding and Unvested as of December 31, 2017 1,498,075 $ 3.41 0.86 RSU Expected to Vest as of December 31, 2017 1,498,075 $ 3.41 0.86 The following table summarizes the Company's Market-Based RSU activity: Number of Weighted Weighted RSU Outstanding as of December 31, 2014 1,769,000 $ 10.67 Granted — — Vested — — Forfeited or Expired — — RSU Outstanding as of December 31, 2015 1,769,000 $ 10.67 Granted — — Vested — — Forfeited or Expired (1,340,000 ) 11.44 RSU Outstanding as of December 31, 2016 429,000 $ 8.26 Granted — — Vested — — Forfeited or Expired (94,500 ) 8.26 RSU Outstanding and Unvested as of December 31, 2017 334,500 $ 8.26 0.09 RSU Expected to Vest as of December 31, 2017 — — 0.00

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of components of income loss before income taxes	The components of loss before income taxes for the years ended December 31, 2015 , 2016 and 2017 are as follows: 2015 2016 2017 U.S. $ (111,437 ) $ 7,150 $ (44,535 ) Foreign (22,407 ) (19,535 ) (38,770 ) Total loss before income taxes $ (133,844 ) $ (12,385 ) $ (83,305 )
Schedule of provision for income taxes	The provision for income taxes consists of the following: 2015 2016 2017 Current: Federal $ 9 $ (226 ) $ 31 State 248 93 231 Foreign 912 567 224 Total current 1,169 434 486 Deferred: Federal 337 478 (978 ) State 71 75 (184 ) Foreign 37 352 (1,238 ) Total deferred 445 905 (2,400 ) Total $ 1,614 $ 1,339 $ (1,914 )
Schedule of reconciliation of federal income tax rate to the actual effective tax rate	Income tax expense (benefit) for the years ended December 31, 2015 , 2016 and 2017 differs from the "expected" amount computed using the federal income tax rate of 35% as a result of the following: 2015 2016 2017 Computed expected tax (benefit) $ (46,846 ) $ (4,335 ) $ (29,157 ) Nondeductible expenses 24,998 5,971 13,420 Tax rate differential on foreign earnings 3,701 720 11,860 Impact of federal income tax rate change — — 59,729 Tax credits (9,988 ) (9,331 ) (27 ) Other (372 ) 833 2,376 Change in valuation allowance 30,121 7,481 (60,115 ) Total tax expense $ 1,614 $ 1,339 $ (1,914 )
Schedule of tax effect of temporary differences that give rise to deferred tax assets and liabilities	The tax effect of temporary differences that give rise to deferred tax assets and liabilities as of December 31, 2016 and 2017 are as follows: 2016 2017 Deferred tax assets: Accrued expenses $ 4,566 $ 5,775 Sales-type leases 55 — Alternative minimum tax and general business credits 6,137 6,291 Stock option expense 26,154 13,782 Other 1,168 881 Loss carryforwards 181,884 103,892 Total deferred tax assets 219,964 130,621 Less valuation allowance (195,968 ) (120,834 ) Net deferred tax assets 23,996 9,787 Deferred tax liabilities: Depreciation and amortization (19,364 ) (3,600 ) Goodwill (5,599 ) (4,206 ) Investments in joint ventures and partnerships (1,432 ) (1,981 ) Total deferred tax liabilities (26,395 ) (9,787 ) Net deferred tax liabilities $ (2,399 ) $ —
Schedule of reconciliation of the total amounts of unrecognized tax benefits	The following is a tabular reconciliation of the total amounts of unrecognized tax benefits for the years ended December 31, 2015 , 2016 and 2017 : Unrecognized tax benefit—December 31, 2015 $ 27,497 Gross increases—tax positions in current year 4,556 Gross increases—tax positions in prior years 17,549 Unrecognized tax benefit—December 31, 2016 49,602 Gross increases—tax positions in current year — Gross decreases—tax positions in prior years (15,537 ) Unrecognized tax benefit—December 31, 2017 $ 34,065

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum lease obligations for all noncancelable operating leases	The following schedule represents the Company's future minimum lease obligations under all noncancelable operating leases as of December 31, 2017 : Fiscal year: 2018 $ 5,626 2019 5,169 2020 4,342 2021 3,183 2022 2,216 Thereafter 13,836 Total future minimum lease payments $ 34,372

Capitalized Lease Obligation 43

Capitalized Lease Obligation and Receivables (Tables)	12 Months Ended
	Dec. 31, 2017
Leases [Abstract]
Schedule of future payments under the capital leases	As of December 31, 2017 , future payments under these capital leases are as follows: 2018 $ 1,374 2019 1,278 2020 1,135 2021 1,044 2022 1,027 Thereafter 2,409 Total minimum lease payments 8,267 Less amount representing interest (1,213 ) Future minimum lease payments 7,054 Less current portion (1,072 ) Capital lease obligations, less current portion $ 5,982
Schedule of future receipts under the leases	As of December 31, 2017 , future receipts under these leases are as follows: 2018 $ 254 2019 186 2020 186 2021 186 2022 186 Thereafter 1,426 Total 2,424 Less amount representing interest (1,231 ) $ 1,193

Reportable Segments and Geogr44

Reportable Segments and Geographic Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule of revenue, operating income (loss), and long-lived assets shown for each geographic area	2015 2016 2017 Revenue: United States $ 330,003 $ 378,497 $ 316,756 Canada 21,818 11,502 6,846 Other 32,499 12,657 17,997 Total revenue $ 384,320 $ 402,656 $ 341,599 Operating loss: United States $ (33,067 ) $ (8,693 ) $ (96,228 ) Canada (4,980 ) (4,212 ) (9,495 ) Other (3,576 ) (4,732 ) (28,724 ) Total operating loss $ (41,623 ) $ (17,637 ) $ (134,447 ) Long-lived assets: United States $ 582,644 $ 547,279 $ 465,245 Canada 68,292 66,191 373 Other 5,693 5,646 — Total long-lived assets $ 656,629 $ 619,116 $ 465,618

Summary of Significant Accoun45

Summary of Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income	$ 1,497	$ 827	$ 843
Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gain from change in fair value of derivatives		22	1,414
Deferred revenue		3,479	$ 4,134
Interest income		$ (843)

Summary of Significant Accoun46

Summary of Significant Accounting Policies - Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Inventories
Raw materials and spare parts	$ 35,145		$ 24,843
Work in process	0		845
Finished goods	93		3,856
Total inventory	$ 35,238		$ 29,544
Station Closures | Discontinued Operations, Held-for-sale or Disposed of by Sale
Divestitures
Inventory, net, current		$ 19,394

Summary of Significant Accoun47

Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Proceeds from grants	$ 4,360	$ 3,295	$ 4,292
LNG liquefaction plants | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
LNG liquefaction plants | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	20 years
Station equipment
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years
Other equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Other equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives	7 years

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Long-Lived Assets (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and intangible assets
Long-lived intangible impairment	$ 32,274,000	$ 32,274,000
Goodwill and intangible asset impairment			$ 0	$ 0
Total intangible assets		13,320,000	85,620,000
Less accumulated amortization		(9,730,000)	(37,591,000)
Foreign currency rate change		0	(9,329,000)
Net intangible assets		3,590,000	38,700,000
Amortization expense		5,065,000	5,794,000	$ 5,539,000
Estimated amortization expense
2,018		1,393,000
2,019		973,000
2,020		765,000
2,021		459,000
2,022		0
Thereafter		0
Technology
Goodwill and intangible assets
Total intangible assets		$ 0	54,400,000
Technology | Minimum
Goodwill and intangible assets
Estimated useful lives		2 years
Technology | Maximum
Goodwill and intangible assets
Estimated useful lives		20 years
Customer relationships
Goodwill and intangible assets
Total intangible assets		$ 5,376,000	16,576,000
Customer relationships | Minimum
Goodwill and intangible assets
Estimated useful lives		1 year
Customer relationships | Maximum
Goodwill and intangible assets
Estimated useful lives		8 years
Acquired contracts
Goodwill and intangible assets
Total intangible assets		$ 4,384,000	4,384,000
Acquired contracts | Minimum
Goodwill and intangible assets
Estimated useful lives		1 year
Acquired contracts | Maximum
Goodwill and intangible assets
Estimated useful lives		10 years
Trademark and trade names
Goodwill and intangible assets
Total intangible assets		$ 2,700,000	8,200,000
Trademark and trade names | Minimum
Goodwill and intangible assets
Estimated useful lives		2 years
Trademark and trade names | Maximum
Goodwill and intangible assets
Estimated useful lives		10 years
Non-compete agreements
Goodwill and intangible assets
Estimated useful lives		3 years
Total intangible assets		$ 860,000	$ 2,060,000
Facility Closing
Goodwill and intangible assets
Long-lived intangible impairment		$ 20,384,000

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Thousands	Dec. 29, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance			$ 93,018	$ 91,967
Foreign currency translation adjustment			1,464	1,051
Goodwill reduced during the year		$ (26,576)	(30,154)
Ending balance			$ 64,328	$ 93,018
SAFE&CEC S.r.l.
Goodwill [Roll Forward]
Goodwill reduced during the year	$ (3,578)

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 341,599	$ 402,656	$ 384,320
Volume -Related
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	264,880	283,814	260,629
Compressor Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	23,527	27,262	54,497
Station Construction Sales
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	51,854	64,942	37,830
AFTC
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	0	26,638	30,986
Other
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Revenues	$ 1,338	$ 0	$ 378

Summary of Significant Accoun51

Summary of Significant Accounting Policies - Alternative Fuels Excise Tax Credit (Details)	12 Months Ended			99 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014$ / gallon
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent)				0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon)				0.50
Subsequent Event [Line Items]
VETC credits recognized as revenue | $	$ 0	$ 26,638,000	$ 30,986,000

Summary of Significant Accoun52

Summary of Significant Accounting Policies - Advertising Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Advertising expense	$ 311	$ 15	$ 44

Summary of Significant Accoun53

Summary of Significant Accounting Policies - Net Loss Per Share (Details) - shares	12 Months Ended			16 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	Oct. 04, 2016
Net Loss Per Share
Common stock, issued (in shares)	145,538,063	151,650,969
GE Credit Agreement | GE Warrant
Net Loss Per Share
Common stock, issued (in shares)					997,740
Common stock | GE Credit Agreement | GE Warrant
Net Loss Per Share
Number of securities called by warrants (in shares)				1,000,000
Number of shares that will not vest and will not become exercisable (in shares)			4,000,000	4,000,000
Stock options
Net Loss Per Share
Anti-dilutive securities (in shares)	8,613,854	11,467,796	11,487,938
Warrants
Net Loss Per Share
Anti-dilutive securities (in shares)	0	0	2,130,682
Convertibles notes
Net Loss Per Share
Anti-dilutive securities (in shares)	14,991,521	16,573,799	35,185,979
Restricted stock units
Net Loss Per Share
Anti-dilutive securities (in shares)	1,832,575	2,072,304	3,419,776

Summary of Significant Accoun54

Summary of Significant Accounting Policies - Foreign Currency Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Foreign exchange transactions gains (losses) included in other income (expense)	$ (246)	$ 132	$ 975

Summary of Significant Accoun55

Summary of Significant Accounting Policies - Concentration of Credit Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Amount in excess of FDIC insurance limits	$ 34,709	$ 34,439

Summary of Significant Accoun56

Summary of Significant Accounting Policies - Recently Adopted Accounting Changes and Recently Issued Accounting Standards (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Jan. 01, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	[1]		$ (303)
Deferred tax assets, valuation allowance		$ 120,834		$ 195,968
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating loss carryforwards			8,600
Deferred tax assets, valuation allowance			8,600
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	[1]		(497)
Accumulated Deficit | Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 303
Accumulated Deficit | Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 194
[1]	(1) The Company adopted two Accounting Standards Updates effective January 1, 2017 (see Note 1 for more information).

Asset Impairments, Other Char57

Asset Impairments, Other Charges, and Inventory Valuation Provision - Summary of Charges (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring and Related Activities [Abstract]
Workforce reduction and related charges			$ 3,057
CEC asset impairments		$ 32,274	32,274
Station closures and related charges			25,557
LCFS Credits charge	$ (7,046)		7,046
Total asset impairments and other charges			67,934	$ 0	$ 0
Inventory valuation provision			13,158	$ 0	$ 0
Total charges			$ 81,092

Asset Impairments, Other Char58

Asset Impairments, Other Charges, and Inventory Valuation Provision (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017USD ($)station	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)station	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 29, 2017USD ($)
Restructuring Cost and Reserve [Line Items]
Asset impairments and other charges			$ 67,934	$ 0	$ 0
Inventory valuation provision			13,158	0	0
Restructuring charges			6,840
CEC asset impairments		$ 32,274	32,274
Impaired long-lived assets	$ 465,618		465,618	$ 619,116	$ 656,629
Customer obligation, LCFS credits, cash payment to settle	7,046		(7,046)
Discontinued Operations, Held-for-sale or Disposed of by Sale | Station Closures
Restructuring Cost and Reserve [Line Items]
Inventory held-for-sale	27,198		27,198
Fair Value, Measurements, Nonrecurring | Level 3
Restructuring Cost and Reserve [Line Items]
Asset impairments and other charges			52,658
Impaired long-lived assets	$ 6,709		6,709			$ 59,367
Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring charges			2,757
Stock-based compensation expense			300
Facility Closing
Restructuring Cost and Reserve [Line Items]
Inventory valuation provision			7,804
CEC asset impairments			20,384
Other restructuring charges			$ 5,173
Facility Closing | Natural Gas Fueling Stations
Restructuring Cost and Reserve [Line Items]
Number of natural gas fueling stations, intended closures | station	42		42
Facility Closing | Natural Gas Fueling Stations | Fair Value, Measurements, Nonrecurring | Level 3
Restructuring Cost and Reserve [Line Items]
Asset impairments and other charges			$ 20,384
Impaired long-lived assets	$ 23,270		23,270
Impaired long-lived assets, fair value	$ 2,886		2,886
Strategic shift in operations
Restructuring Cost and Reserve [Line Items]
Inventory valuation provision			$ 5,354

Asset Impairments, Other Char59

Asset Impairments, Other Charges, and Inventory Valuation Provision - Cash Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Restructuring Cost and Reserve [Line Items]
Charges	$ 6,840
Cash Payments Made in the Year Ended December 31, 2017	(2,827)
Restructuring Reserve	4,013
Employee severance
Restructuring Cost and Reserve [Line Items]
Charges	2,757
Cash Payments Made in the Year Ended December 31, 2017	(2,757)
Restructuring Reserve	0
Lease termination fees and AROs for station closures
Restructuring Cost and Reserve [Line Items]
Charges	4,083
Cash Payments Made in the Year Ended December 31, 2017	(70)
Restructuring Reserve	$ 4,013

Acquisitions and Divestitures -

Acquisitions and Divestitures - BP Transaction (Details)	Jun. 22, 2017USD ($)	Mar. 31, 2017USD ($)	Feb. 27, 2017USD ($)facility	Dec. 31, 2017USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Acquisitions and Divestitures
Gain from sale of certain assets of subsidiary					$ 70,658,000	$ 0	$ 937,000
Goodwill				$ 64,328,000	64,328,000	$ 93,018,000	$ 91,967,000
BP Products North America, Inc.
Acquisitions and Divestitures
Production facilities to be developed | facility			2
Asset purchase agreement, amount		$ 155,511,000
Extinguishment of debt, amount		8,820,000
Asset purchase agreement, cash paid		30,000,000		1,007,000
Asset purchase agreement, debt instrument, face amount		123,487,000
Asset purchase agreement, post-closing adjustments, payment	$ 2,010,000
Asset purchase agreement, contingent consideration (up to)			$ 25,000		772,462
Asset purchase agreement, contingent consideration, term			5 years
Asset purchase agreement, payment of transaction costs		$ 3,695,000		$ 8,605,000
Asset Purchase agreement, shares issued (in shares) | shares				770,269
Asset purchase agreement, shares issued, value				$ 1,964,000
Proceeds from sale of other assets				142,190,000
Gain from sale of certain assets of subsidiary					70,658,000
Goodwill				$ 26,576,000	$ 26,576,000

Acquisitions and Divestitures61

Acquisitions and Divestitures - SAFE&CEC S.r.l. (Details) - USD ($) $ in Thousands	Dec. 29, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 26, 2017
Acquisitions and Divestitures
Working capital adjustments, funding for certain post-closing commitments, and transaction fees			$ 13,783	$ 8,073
Gain from sale of certain assets of subsidiary			70,658	0	$ 937
Goodwill reduced during the year		$ 26,576	30,154
Loss before income taxes			83,305	12,385	133,844
CEC
Acquisitions and Divestitures
Loss before income taxes			45,126	$ 15,601	$ 13,657
SAFE&CEC S.r.l.
Acquisitions and Divestitures
Ownership interest	49.00%					49.00%
Investment balance			27,883
Working capital adjustments, funding for certain post-closing commitments, and transaction fees			3,986
Gain from sale of certain assets of subsidiary			$ 6,465
Goodwill reduced during the year	$ 3,578
SAFE&CEC S.r.l. | Landi Renzo S.p.A.
Acquisitions and Divestitures
Ownership interest						51.00%

Restricted Cash (Details)

Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash
Short-term restricted cash	$ 1,127	$ 6,996
Canton Bonds
Restricted Cash
Short-term restricted cash	0	3,665
Held in escrow
Restricted Cash
Short-term restricted cash	0	1,578
Standby letters of credit
Restricted Cash
Short-term restricted cash	$ 1,127	$ 1,753

Investments (Details)

Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term investments
Amortized Cost	$ 141,584	$ 73,729
Gross Unrealized Losses	(122)	(11)
Estimated Fair Value	141,462	73,718
Municipal bonds and notes
Short-term investments
Amortized Cost	21,414	8,791
Gross Unrealized Losses	(49)	(4)
Estimated Fair Value	21,365	8,787
Zero coupon bonds
Short-term investments
Amortized Cost	54,159
Gross Unrealized Losses	(33)
Estimated Fair Value	54,126
Corporate bonds
Short-term investments
Amortized Cost	55,109	21,517
Gross Unrealized Losses	(40)	(7)
Estimated Fair Value	55,069	21,510
Certificate of deposits
Short-term investments
Amortized Cost	10,902	43,421
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 10,902	$ 43,421

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 29, 2017
Liabilities:
Impaired long-lived assets	$ 465,618	$ 619,116	$ 656,629
Asset impairments and other charges	67,934	0	$ 0
Fair value measured on recurring basis | Warrants
Liabilities:
Warrants	536	581
Fair value measured on recurring basis | Level 1 | Warrants
Liabilities:
Warrants	0	0
Fair value measured on recurring basis | Level 2 | Warrants
Liabilities:
Warrants	0	0
Fair value measured on recurring basis | Level 3 | Warrants
Liabilities:
Warrants	536	581
Fair value measured on recurring basis | Municipal bonds and notes
Assets:
Available-for-sale securities	21,365	8,787
Fair value measured on recurring basis | Municipal bonds and notes | Level 1
Assets:
Available-for-sale securities		0
Fair value measured on recurring basis | Municipal bonds and notes | Level 2
Assets:
Available-for-sale securities	21,365	8,787
Fair value measured on recurring basis | Municipal bonds and notes | Level 3
Assets:
Available-for-sale securities	0	0
Fair value measured on recurring basis | Zero coupon bonds
Assets:
Available-for-sale securities	54,126
Fair value measured on recurring basis | Zero coupon bonds | Level 1
Assets:
Available-for-sale securities
Fair value measured on recurring basis | Zero coupon bonds | Level 2
Assets:
Available-for-sale securities	54,126
Fair value measured on recurring basis | Zero coupon bonds | Level 3
Assets:
Available-for-sale securities	0
Fair value measured on recurring basis | Corporate bonds
Assets:
Available-for-sale securities	55,069	21,510
Fair value measured on recurring basis | Corporate bonds | Level 1
Assets:
Available-for-sale securities		0
Fair value measured on recurring basis | Corporate bonds | Level 2
Assets:
Available-for-sale securities	55,069	21,510
Fair value measured on recurring basis | Corporate bonds | Level 3
Assets:
Available-for-sale securities	0	0
Fair value measured on recurring basis | Certificate of deposits
Assets:
Available-for-sale securities	10,902	43,421
Fair value measured on recurring basis | Certificate of deposits | Level 1
Assets:
Available-for-sale securities		0
Fair value measured on recurring basis | Certificate of deposits | Level 2
Assets:
Available-for-sale securities	10,902	43,421
Fair value measured on recurring basis | Certificate of deposits | Level 3
Assets:
Available-for-sale securities	0	$ 0
Fair Value, Measurements, Nonrecurring | Level 3
Liabilities:
Impaired long-lived assets	6,709			$ 59,367
Asset impairments and other charges	$ 52,658

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Recognition (Details) - Warrants - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the beginning and ending balances of items measured at fair value using significant unobservable inputs (Level 3)
Beginning Balance	$ 581	$ 632
Gain included in earnings	(45)	(51)
Ending Balance	$ 536	$ 581

Other Receivables (Details)

Other Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Receivables
Other receivables	$ 19,235	$ 21,934
Loans to customers to finance vehicle purchases
Other Receivables
Other receivables	4,746	7,416
Accrued customer billings
Other Receivables
Other receivables	10,072	4,308
Fuel tax credits
Other Receivables
Other receivables	177	6,358
Other
Other Receivables
Other receivables	$ 4,240	$ 3,852

Land, Property and Equipment (D

Land, Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
Land, Property and Equipment
Land, property and equipment, gross	$ 635,706	$ 752,407
Less accumulated depreciation	(268,401)	(268,484)
Total land, property and equipment, net	367,305	483,923
Capitalized software costs, net	26,003	25,728
Accumulated amortization on the capitalized software costs	18,737	17,237
Amortization expense related to the capitalized software costs	4,382	3,444	$ 3,053
Amount included in accounts payable balances	4,377	4,053
Discontinued Operations, Held-for-sale or Disposed of by Sale | Station Closures
Land, Property and Equipment
Inventory, net, current				$ 19,394
Land
Land, Property and Equipment
Land, property and equipment, gross	2,858	2,858
LNG liquefaction plants
Land, Property and Equipment
Land, property and equipment, gross	94,634	94,634
RNG plants
Land, Property and Equipment
Land, property and equipment, gross	0	47,545
Station equipment
Land, Property and Equipment
Land, property and equipment, gross	304,090	341,605
Trailers
Land, Property and Equipment
Land, property and equipment, gross	70,906	54,985
Other equipment
Land, Property and Equipment
Land, property and equipment, gross	88,313	93,118
Construction in progress
Land, Property and Equipment
Land, property and equipment, gross	$ 74,905	$ 117,662

Investments in Other Entities68

Investments in Other Entities and Noncontrolling Interest in a Subsidiary (Details) - USD ($) $ in Thousands	Jul. 14, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 29, 2017	Nov. 26, 2017	Oct. 14, 2014	Sep. 16, 2014
Equity method investments
Loss from investment		$ (131)	$ (22)	$ (815)
Capital from equity method investment		0	3,031	0
Loss attributable to noncontrolling interest		2,154	1,571	1,216
SAFE&CEC S.r.l.
Equity method investments
Ownership interest					49.00%	49.00%
Investment balance		27,883
NG Advantage
Equity method investments
Loss attributable to noncontrolling interest		2,154	1,571	1,216
Noncontrolling interest		$ 22,668	24,822
NG Advantage | Preferred Stock
Equity method investments
Subsidiary equity issuance	$ 7,500
Common unit purchase agreement | NG Advantage
Equity method investments
Ownership interest acquired							53.30%
RNG Ventures
Equity method investments
Percentage of right to purchase products for vehicle fuels market		100.00%
Investment balance		$ 0	833
Mansfield Clean Energy Partners LL
Equity method investments
Ownership interest								50.00%
Investment balance		1,512	1,642
Loss from investment		$ 131	22	$ 815
Capital from equity method investment			$ 3,031

Accrued Liabilities (Details)

Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Accrued alternative fuels incentives	$ 2,954	$ 9,840
Accrued employee benefits	2,378	4,317
Accrued interest	1,486	1,849
Accrued gas and equipment purchases	8,722	11,657
Accrued property and other taxes	4,582	4,572
Salaries and wages	8,363	12,293
Other	13,783	8,073
Total accrued liabilities	$ 42,268	$ 52,601

Debt - Debt and Capital Lease O

Debt - Debt and Capital Lease Obligations (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Feb. 29, 2016	Sep. 30, 2013	Jul. 11, 2011
Long-term debt
Principal Balances	$ 260,931	$ 314,348
Unamortized Debt Financing Costs	844	1,972
Balance, Net of Financing Costs	260,087	312,376
Principal Balances, amounts due within one year	(140,223)	(6,126)
Unamortized Debt Financing Costs, amounts due within one year	(524)	(183)
Balance, Net of Financing Costs, amounts due within one year	(139,699)	(5,943)
Principal Balances, long-term	120,708	308,222
Unamortized Debt Financing Costs, long-term	320	1,789
Balance, Net of Financing Costs, long-term	$ 120,388	306,433
Interest rate	5.02%
Summary of aggregate maturities of long-term debt
2,018	$ 140,223
2,019	54,492
2,020	54,578
2,021	4,173
2,022	3,797
Thereafter	3,668
Long-term debt, related party	0	65,000
Mr. Pickens
Summary of aggregate maturities of long-term debt
Long-term debt, related party	0	65,000
7.5% Notes
Long-term debt
Principal Balances	125,000	150,000
Unamortized Debt Financing Costs	131	274
Balance, Net of Financing Costs	$ 124,869	149,726
Interest rate	7.50%				7.50%
Summary of aggregate maturities of long-term debt
2,018	$ 25,000
2,019	50,000
2,020	50,000
2,021	0
2,022	0
Thereafter	0
5.25% Notes
Long-term debt
Principal Balances	110,450	110,450
Unamortized Debt Financing Costs	454	1,088
Balance, Net of Financing Costs	109,996	109,362
Interest rate				5.25%
Summary of aggregate maturities of long-term debt
2,018	110,450
2,019	0
2,020	0
2,021	0
2,022	0
Thereafter	0
PlainsCapital Bank
Long-term debt
Principal Balances		23,500
Unamortized Debt Financing Costs		0
Balance, Net of Financing Costs		23,500
Interest rate			2.70%
Canton Bonds
Long-term debt
Principal Balances		9,520
Unamortized Debt Financing Costs		373
Balance, Net of Financing Costs		9,147
NG Advantage debt
Long-term debt
Principal Balances	17,185
Unamortized Debt Financing Costs	259
Balance, Net of Financing Costs	16,926
Summary of aggregate maturities of long-term debt
2,018	3,483
2,019	3,242
2,020	3,413
2,021	3,055
2,022	2,665
Thereafter	1,327
Capital lease obligations
Long-term debt
Principal Balances	7,054	6,028
Unamortized Debt Financing Costs	0	0
Balance, Net of Financing Costs	7,054	6,028
Summary of aggregate maturities of long-term debt
2,018	1,072
2,019	1,021
2,020	925
2,021	871
2,022	917
Thereafter	2,248
Other debt
Long-term debt
Principal Balances	1,242	14,850
Unamortized Debt Financing Costs	0	237
Balance, Net of Financing Costs	1,242	$ 14,613
Summary of aggregate maturities of long-term debt
2,018	218
2,019	229
2,020	240
2,021	247
2,022	215
Thereafter	$ 93

Debt - 7.5% Notes (Details)

Debt - 7.5% Notes (Details)	Feb. 09, 2017USD ($)	Jun. 14, 2013USD ($)day$ / sharesshares	Jul. 11, 2011USD ($)note	Dec. 31, 2017USD ($)	Nov. 17, 2017USD ($)	Feb. 21, 2017USD ($)	Aug. 27, 2013USD ($)
Long-term debt
Interest rate				5.02%
7.5% Notes
Long-term debt
Interest rate			7.50%	7.50%
Financing commitment received (up to)			$ 150,000,000
Number of convertible promissory notes to be issued | note			3
Financing commitment issuance period			3 years
Principal amount of each debt instrument to be issued			$ 50,000,000
7.5% Notes | Buyers
Long-term debt
Additional amount of advances under the obligation assumed		$ 50,000,000
Amount of advance funded		$ 50,000,000
Conversion price of shares (in dollars per share) | $ / shares		$ 15.80
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion		40.00%
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion | day		20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion		30 days
Period during which the debt instrument principal balance is required to be paid following its issuance		7 years
Repayment in shares at maturity covenant, maximum number of shares (in shares) | shares		13,993,630
7.5% Notes | Green Energy Investment Holdings, LLC
Long-term debt
Principal amount transferred						$ 11,800,000	$ 16,800,000
Aggregate principal amount							68,200,000
7.5% Notes | Green Energy Investment Holdings, LLC | Mr. Pickens
Long-term debt
Principal amount transferred					$ 40,000,000
Repurchased face amount during period	$ 25,000,000
Repayments of convertible debt	$ 21,750,000
Gain on repurchase of debt instrument				$ 3,191,000
7.5% Notes | Other Third Parties
Long-term debt
Aggregate principal amount							$ 56,800,000

Debt - SLG Notes (Details)

Debt - SLG Notes (Details) - USD ($)	Jul. 14, 2016	Feb. 29, 2016	Dec. 31, 2016	Dec. 31, 2017	Mar. 01, 2012	Aug. 24, 2011
Long-term debt
Interest rate				5.02%
SLG Notes
Long-term debt
Financing commitment received						$ 150,000,000
Interest rate						7.50%
Principal amount transferred					$ 24,000,000
Repayments of debt		$ 60,000,000
Repayments of debt, accrued interest		$ 1,812,000
SLG Notes | Purchaser
Long-term debt
Repayments of debt, accrued interest	$ 248,000
Debt instrument, repurchased face amount	85,000,000
Repayments of convertible debt	$ 38,155,000
Loss on repurchase of debt instrument			$ 891,000
SLG Notes | Purchaser | Common stock
Long-term debt
Common stock issued upon conversion of debt (in shares)	14,000,000

Debt - 5.25% (Details)

Debt - 5.25% (Details)	May 04, 2016USD ($)shares	Sep. 30, 2013USD ($)day$ / sharesshares	Dec. 31, 2016USD ($)	Dec. 31, 2017
Long-term debt
Interest rate				5.02%
5.25% Notes
Long-term debt
Interest rate		5.25%
Debt issuance amount		$ 250,000,000
Payment of certain debt issuance costs		7,805,000
Amount of advance funded		242,195,000
Denominator in calculation of shares per principal amount in debt conversion		$ 1,000
Common stock issued upon conversion of debt (in shares) | shares	6,265,829	64.1026
Conversion price of shares (in dollars per share) | $ / shares		$ 15.60
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion | day		20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion		30 days
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion		160.00%
Redemption price as percentage of principal amount of notes to be redeemed		100.00%
Percentage of principal amount at which notes may be required to be repurchased in event of fundamental change by the entity		100.00%
Repayments of convertible debt			$ 84,344,000
Repurchased face amount during period			114,550,000
Repayments of debt, accrued interest			1,546,000
Converted instrument, amount	$ 25,000,000
Gain on repurchase of debt instrument			$ 35,239,000
5.25% Notes | Minimum
Long-term debt
Percentage of principal amount of notes outstanding allowing holders to accelerate all amounts due under notes in event of default under the Indenture		25.00%

Debt - PlainsCapital Bank Credi

Debt - PlainsCapital Bank Credit Facility (Details) - USD ($)	Feb. 29, 2016	Dec. 31, 2017
Long-term debt
Interest rate		5.02%
Plains Capital Bank Credit Agreement
Long-term debt
Line of credit limit (up to)	$ 50,000,000
Period during which the debt instrument principal balance is required to be paid following its issuance	1 year
Draw down of line of credit	$ 50,000,000	$ 23,500,000
Interest rate	2.70%
Plains Capital Bank Credit Agreement | London Interbank Offered Rate (LIBOR)
Long-term debt
Basis spread on variable rate	2.30%

Debt - Canton Bonds and Other D

Debt - Canton Bonds and Other Debt (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Mar. 19, 2014
Long-term debt
Effective interest rate (up to)	5.02%
Weighted average interest rate	4.79%	5.72%
Canton Bonds | Canton Renewables
Long-term debt
Debt issuance amount			$ 12,400,000

Debt - NG Advantage Debt (Detai

Debt - NG Advantage Debt (Details)	Dec. 31, 2017USD ($)	Jan. 24, 2017USD ($)installment	Dec. 31, 2016USD ($)	Nov. 30, 2016USD ($)installment	May 12, 2016USD ($)
Long-term debt
Interest rate	5.02%
Weighted average interest rate	4.79%		5.72%
Outstanding principal balance	$ 260,087,000		$ 312,376,000
Trailer And Equipment Debt
Long-term debt
Weighted average interest rate	4.87%		5.51%
Outstanding principal balance	$ 9,207,000		$ 2,598,000
Trailer And Equipment Debt | Maximum
Long-term debt
Interest rate	6.01%
Commerce | Loan And Security Agreement | 4.41% Term Loan
Long-term debt
Debt issuance amount					$ 6,300,000
Interest rate					4.41%
Commerce | Loan And Security Agreement | 5.0% Term Loan
Long-term debt
Debt issuance amount		$ 6,150,000
Number of monthly installments | installment		84
Interest rate		5.00%
Wintrust USA | Loan And Security Agreement | 5.17% Term Loan
Long-term debt
Debt issuance amount				$ 4,695,000
Number of monthly installments | installment				72
Interest rate				5.17%

Debt - Termination of GE Credit

Debt - Termination of GE Credit Agreement (Details) - GE Credit Agreement	Nov. 07, 2012USD ($)subsidiaryplant$ / sharesshares	Dec. 31, 2015USD ($)shares
Long-term debt
Number of wholly owned subsidiaries through which the entity entered into a financing arrangement | subsidiary	2
Line of credit limit (up to) | $	$ 200,000,000
Number of LNG production facilities being financed | plant	2
Write off of deferred financing costs | $		$ 54,925,000
GE Warrant | Common stock
Long-term debt
Number of shares that can be purchased upon exercise of warrants (in shares) | shares	5,000,000	1,000,000
Exercise price of the warrant (in dollars per share) | $ / shares	$ 0.01
Number of shares that will not vest and will not become exercisable (in shares) | shares		4,000,000

Stockholders' Equity (Details)

Stockholders' Equity (Details)	Dec. 31, 2017classvote$ / sharesshares	Dec. 31, 2016$ / sharesshares
Stockholders' Equity Note [Abstract]
Authorized number of classes of capital stock | class	2
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Authorized shares (in shares)	225,000,000
Common stock, authorized (in shares)	224,000,000	224,000,000
Preferred stock, authorized (in shares)	1,000,000	1,000,000
Number of votes per share held by holders of common stock | vote	1

Stockholders' Equity - Issuance

Stockholders' Equity - Issuance of Common Stock and Warrants (Details) - USD ($)	Nov. 07, 2012	Nov. 30, 2008	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	May 31, 2017	Oct. 04, 2016
Stockholders' equity
Common stock, issued (in shares)					145,538,063	151,650,969
ATM Program Activity
Fees and issuance costs					$ 638,000	$ 2,283,000	$ 883,000
Sales Agreement | Citigroup
Stockholders' equity
Common stock, amount authorized (up to)									$ 200,000
ATM Program Activity
Gross proceeds			$ 6,943,000		10,767,000	103,591,000
Fees and issuance costs			493,000		311,000	2,612,000
Net proceeds			$ 6,450,000		$ 10,456,000	$ 100,979,000
Shares issued (in shares)			1,561,902		3,802,500	31,064,434
GE Credit Agreement
Stockholders' equity
Write off of deferred financing costs				$ 54,925,000
Series I Warrants
Stockholders' equity
Period from which warrants are exercisable		6 months
Warrant expiration term		7 years
Exercise price of the warrant (in dollars per share)		$ 12.54
Series I Warrants | Maximum
Stockholders' equity
Number of shares issued for units under Placement Agent Agreement (in shares)		4,419,192
Number of shares that can be purchased upon exercise of warrants (in shares)		3,314,394
GE Warrant | GE Credit Agreement
Stockholders' equity
Warrants outstanding, not exercisable (in shares)						4,000,000
Common stock, issued (in shares)										997,740
Fair value of warrants issued, additional paid-in capital	$ 56,158,000
GE Warrant | Common stock | GE Credit Agreement
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants (in shares)	5,000,000		1,000,000	1,000,000			1,000,000	1,000,000
Exercise price of the warrant (in dollars per share)	$ 0.01
Number of securities called by warrants (in shares)								1,000,000
NG Advantage Warrants | NG Advantage
ATM Program Activity
Fair value of warrants outstanding					$ 536,000	$ 581,000
Gain (loss) from the change in fair value					$ 45,000	$ (21,000)	$ 69,000
NG Advantage Warrants | Common stock | NG Advantage
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants (in shares)					127,200

Stockholders' Equity - Stock-Ba

Stockholders' Equity - Stock-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation
Stock-based compensation expense, net of $0 tax in 2015, 2016 and 2017	$ 8,423,000	$ 8,092,000	$ 10,779,000
Stock-based compensation expense, tax	$ 0	$ 0	$ 0
Weighted-average assumption used for grants
Expected volatility	63.61%
Risk-free interest rate	2.05%
2006 Plan | Stock options
Stock-based compensation
Number of shares available for future grant	4,823,956
Number of Shares
Outstanding at the beginning of the period (in shares)	11,467,796	11,487,938	11,486,301
Granted (in shares)	1,139,500	284,750	1,415,200
Exercised (in shares)	0	0	(608,279)
Forfeited or expired (in shares)	(3,993,442)	(304,892)	(805,284)
Outstanding at the end of the period (in shares)	8,613,854	11,467,796	11,487,938
Exercisable at the end of the period (in shares)	7,118,956
Vested and expected to vest at the end of the period (in shares)	8,613,854
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 11.25	$ 11.44	$ 11.91
Granted (in dollars per share)	2.83	3.63	5.39
Exercised (in dollars per share)	0	0	2.96
Forfeited or expired (in dollars per share)	12.34	11.30	14.04
Outstanding at the end of the period (in dollars per share)	9.62	$ 11.25	$ 11.44
Exercisable at the end of the period (in dollars per share)	10.91
Vested and expected to vest at the end of the period (in dollars per share)	$ 9.62
Additional option disclosures
Outstanding at the end of the period, weighted average remaining contractual term	4 years 7 months 24 days
Exercisable at the end of the period, weighted average remaining contractual term	3 years 9 months 22 days
Vested and expected to vest at the end of the period, weighted average remaining contractual term	4 years 7 months 24 days
Outstanding at the end of the period, aggregate intrinsic value	$ 0
Exercisable at the end of the period, aggregate intrinsic value	0
Vested and expected to vest at the end of the period, aggregate intrinsic value	0
Total unrecognized compensation cost related to non-vested shares	$ 2,076,000
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized	1 year 6 months 4 days
Total fair value of shares vested	$ 2,463,000
Intrinsic value of all options exercised	$ 0	$ 0	$ 2,197,000
Weighted-average assumption used for grants
Dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum		61.10%	59.20%
Expected volatility, maximum		70.80%	72.00%
Risk-free interest rate, minimum		1.20%	1.70%
Risk-free interest rate, maximum		2.00%	1.80%
Expected life	6 years	6 years	6 years
Stock-based compensation
Weighted-average grant date fair values of options granted (in dollars per share)	$ 1.67	$ 2.30	$ 3.29
Stock-based compensation expense	$ 2,213,000	$ 2,561,000	$ 5,195,000
Employee Severance
Stock-based compensation
Stock-based compensation expense, net of $0 tax in 2015, 2016 and 2017	300,000
Stock-based compensation
Stock-based compensation expense	$ 300,000

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Stock Units (Details) - USD ($)	12 Months Ended					72 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2017
Market-Based RSUs
Stock-based compensation
Number of shares of common stock to be received for each performance unit (in shares)	1
Period from the date of grant after which awards will be automatically forfeited if the Stock Price Condition is not satisfied	4 years
Shares forfeited (in shares)						1,700,000
Number of Shares
Outstanding at the beginning of the period (in shares)	429,000	1,769,000	1,769,000
Granted (in shares)	0	0	0
Vested (in shares)	0	0	0
Forfeited or expired (in shares)	(94,500)	(1,340,000)	0
Outstanding at the end of the period (in shares)	334,500	429,000	1,769,000	1,769,000		334,500
Expected to vest at the end of the period (in shares)	0					0
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)	$ 8.26	$ 10.67	$ 10.67
Granted (in dollars per share)	0	0	0
Vested (in dollars per share)	0	0	0
Forfeited or expired (in dollars per share)	8.26	11.44	0
Outstanding at the end of the period (in dollars per share)	8.26	$ 8.26	$ 10.67	$ 10.67		$ 8.26
Expected to vest at the end of the period (in dollars per share)	$ 0					$ 0
Outstanding at the end of the period, weighted average remaining contractual term	1 month 2 days
Expected to vest at the end of the period, weighted average remaining contractual term	0 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 0					$ 0
Stock-based compensation expense	$ 0	$ 169,000	$ 1,770,000
Market-Based RSUs | Minimum
Stock-based compensation
Period from the date of grant in which the closing price of the entity's common stock must exceed the closing price in order for a holder to receive one share of common stock for each award	2 years
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the closing price on the grant date in order for a holder to receive one share of common stock for each award	20 days
Percentage of the closing price of the entity's common stock that the closing price must equal or exceed in order for an award holder to receive one share of common stock for each award	135.00%
Market-Based RSUs | Maximum
Stock-based compensation
Period from the date of grant in which the closing price of the entity's common stock must exceed the closing price in order for a holder to receive one share of common stock for each award	4 years
Service-Based RSUs
Number of Shares
Outstanding at the beginning of the period (in shares)	1,643,304	1,650,776	822,752
Granted (in shares)	2,835,331	850,125	1,167,750
Vested (in shares)	(2,840,584)	(726,687)	(283,726)
Forfeited or expired (in shares)	(139,976)	(130,910)	(56,000)
Outstanding at the end of the period (in shares)	1,498,075	1,643,304	1,650,776	822,752		1,498,075
Expected to vest at the end of the period (in shares)	1,498,075					1,498,075
Weighted Average Fair Value at Grant Date
Outstanding at the beginning of the period (in dollars per share)	$ 4.56	$ 5.50	$ 5.82
Granted (in dollars per share)	1.36	3.63	5.38
Vested (in dollars per share)	1.97	5.53	5.94
Forfeited or expired (in dollars per share)	4.69	4.91	5.57
Outstanding at the end of the period (in dollars per share)	3.41	$ 4.56	$ 5.50	$ 5.82		$ 3.41
Expected to vest at the end of the period (in dollars per share)	$ 3.41					$ 3.41
Outstanding at the end of the period, weighted average remaining contractual term	10 months 10 days
Expected to vest at the end of the period, weighted average remaining contractual term	10 months 10 days
Stock-based compensation
Total unrecognized compensation cost related to non-vested shares	$ 3,145,000					$ 3,145,000
Weighted average period over which the total unrecognized compensation cost related to non-vested shares is expected to be recognized	1 year 6 months 22 days
Stock-based compensation expense	$ 5,901,000	$ 4,395,000	$ 2,622,000
Vesting period	3 years
Service-Based RSUs | Vesting over the first year
Stock-based compensation
Vesting percentage	34.00%
Service-Based RSUs | Vesting over the second year
Stock-based compensation
Vesting percentage	33.00%
Service-Based RSUs | Vesting over the third year
Stock-based compensation
Vesting percentage	33.00%
2012 and 2014 | Market-Based RSUs
Number of Shares
Granted (in shares)				2,034,500	2,034,500

Stockholders' Equity - Employee

Stockholders' Equity - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan $ in Thousands	May 07, 2013period	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2017shares
Stock-based compensation
Purchase price of shares expressed as percentage of fair market value of common stock	85.00%
Number of non-concurrent offering periods | period	2
Number of shares reserved (in shares)		2,500,000			2,500,000
Stock-based compensation expense | $		$ 41	$ 51	$ 50
Shares sold pursuant to the ESPP (in shares)					319,460

Stockholders' Equity - Non-Qual

Stockholders' Equity - Non-Qualified Non-Public Subsidiary Unit Options (Details) - CERF Plan - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Outstanding at the beginning of the period (in shares)		108,000	108,000	115,000
Granted (in shares)		0	0	0
Exercised (in shares)		0	0	0
Forfeited or expired (in shares)		(108,000)	0	(7,000)
Outstanding at the end of the period (in shares)		0	108,000	108,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 40.80	$ 40.80	$ 40.80
Granted (in dollars per share)		0	0	0
Exercised (in dollars per share)		0	0	0
Forfeited or expired (in dollars per share)		40.80	0	40.80
Outstanding at the end of the period (in dollars per share)			$ 40.80	$ 40.80
Additional option disclosures
Outstanding at the end of the period, weighted average remaining contractual term
Outstanding at the end of the period, aggregate intrinsic value
Stock-based compensation
Weighted-average grant date fair values of options granted (in dollars per share)	$ 31.65
Stock options
Stock-based compensation
Number of shares reserved for issuance (in shares)	150,000
Stock-based compensation
Stock-based compensation expense		$ 0	$ 803	$ 1,115

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of loss before income taxes
U.S.	$ (44,535)	$ 7,150	$ (111,437)
Foreign	(38,770)	(19,535)	(22,407)
Total loss before income taxes	(83,305)	(12,385)	(133,844)
Current:
Federal	31	(226)	9
State	231	93	248
Foreign	224	567	912
Total current	486	434	1,169
Deferred:
Federal	(978)	478	337
State	(184)	75	71
Foreign	(1,238)	352	37
Total deferred	(2,400)	905	445
Income tax expense (benefit)	(1,914)	$ 1,339	$ 1,614
Federal income tax expense (benefit), utilization of operating loss carryforwards	6,002
State and local income tax expense (benefit), utilization of operating loss carryforwards	$ 1,328
Federal income tax rate	35.00%	35.00%	35.00%
Reconciliation of the income tax provision
Computed expected tax (benefit)	$ (29,157)	$ (4,335)	$ (46,846)
Nondeductible expenses	13,420	5,971	24,998
Tax rate differential on foreign earnings	11,860	720	3,701
Impact of federal income tax rate change	59,729	0	0
Tax credits	(27)	(9,331)	(9,988)
Other	2,376	833	(372)
Change in valuation allowance	(60,115)	7,481	30,121
Income tax expense (benefit)	(1,914)	1,339	1,614
Tax benefit related to exclusion of AFTC	0	9,112	$ 9,298
Deferred tax assets:
Accrued expenses	5,775	4,566
Sales-type leases	0	55
Alternative minimum tax and general business credits	6,291	6,137
Stock option expense	13,782	26,154
Other	881	1,168
Loss carryforwards	103,892	181,884
Total deferred tax assets	130,621	219,964
Less valuation allowance	(120,834)	(195,968)
Net deferred tax assets	9,787	23,996
Deferred tax liabilities:
Depreciation and amortization	(3,600)	(19,364)
Goodwill	(4,206)	(5,599)
Investments in joint ventures and partnerships	(1,981)	(1,432)
Total deferred tax liabilities	(9,787)	(26,395)
Net deferred tax liabilities	$ 0	$ (2,399)

Income Taxes - Tax Credit Carry

Income Taxes - Tax Credit Carryforwards (Details) $ in Thousands	Dec. 31, 2017USD ($)
Federal
Income Taxes
Operating loss carryforwards	$ 417,402
Federal | General
Income Taxes
Tax credit carryforwards	6,083
State
Income Taxes
Operating loss carryforwards	284,955
Foreign
Income Taxes
Operating loss carryforwards	$ 727

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation allowance
Deferred tax assets, valuation allowance	$ 120,834	$ 195,968
Net increase in valuation allowance	(75,134)	6,765
Unrecognized tax benefits, which if recognized, would primarily affect the effective tax rate	692
Unrecognized tax benefits, which if recognized, would increase the net operating loss carryforwards	33,373
Reconciliation of the total amounts of unrecognized tax benefits
Unrecognized tax benefit at the beginning of the period	49,602	27,497
Gross increases—tax positions in current year	0	4,556
Gross increases—tax positions in prior years		17,549
Gross decreases—tax positions in prior years	(15,537)
Unrecognized tax benefit at the end of the period	34,065	49,602	$ 27,497
Additional information on income tax
Accrued interest expense and penalties	308	241
Interest expense related to uncertain tax positions	$ 67	$ 62	$ 58

Commitments and Contingencies87

Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fiscal year:
2,018	$ 5,626
2,019	5,169
2,020	4,342
2,021	3,183
2,022	2,216
Thereafter	13,836
Total future minimum lease payments	34,372
Rent expense	$ 7,878	$ 11,058	$ 8,629

Commitments and Contingencies -

Commitments and Contingencies - Long-Term Take-or-Pay Natural Gas Supply Contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
LNG supply agreement | DGS
Long-Term Take-or-pay Contracts
Amount paid under the contract	$ 8,092	$ 9,692	$ 11,852
Fixed commitments under the contract payable in future
2,018	4,818
2,019	4,818
2,020	1,201
CNG supply agreement | JTA
Fixed commitments under the contract payable in future
2,018	313
2,019	429
2,020	$ 548

Commitments and Contingencies89

Commitments and Contingencies - Long-Term Natural Gas Purchase Contracts (Details) - Long-Term Natural Gas Purchase Contracts - USD ($) $ in Thousands	1 Months Ended
	Jun. 30, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Term of agreement	5 years
Nonrefundable amount	$ 13,360
Due in 2018		$ 2,593
Due in 2019		13,157
Due in 2020		13,315
Due in 2021		13,317
Due in 2022		$ 10,671

Commitments and Contingencies90

Commitments and Contingencies - Purchase of Natural Gas Heavy-Duty Trucks (Details) $ in Thousands	Dec. 31, 2017USD ($)truck	Jul. 31, 2017truck
Commitments and Contingencies Disclosure [Abstract]
Asset purchase arrangement, number of trucks | truck	124	146
Asset purchase arrangement, value of trucks | $	$ 7,936

Capitalized Lease Obligation 91

Capitalized Lease Obligation and Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Weighted average interest rate	6.59%
Future payments under these capital leases
2,018	$ 1,374
2,019	1,278
2,020	1,135
2,021	1,044
2,022	1,027
Thereafter	2,409
Total minimum lease payments	8,267
Less amount representing interest	(1,213)
Future minimum lease payments	7,054
Less current portion	(1,072)
Capital lease obligations, less current portion	5,982
Value of the equipment under capital lease	7,934	$ 10,168
Accumulated amortization	$ 846	$ 4,073

Capitalized Lease Obligation 92

Capitalized Lease Obligation and Receivables - Future Receipts (Details) $ in Thousands	Dec. 31, 2017USD ($)
Leases [Abstract]
Sales-type leases interest rate	13.10%
Future receipts under the leases
2,018	$ 254
2,019	186
2,020	186
2,021	186
2,022	186
Thereafter	1,426
Total	2,424
Less amount representing interest	(1,231)
Present value of future minimum lease receipts	$ 1,193

401(k) Plan (Details)

401(k) Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Maximum percentage of base pay that can be contributed by employees through salary deferrals	90.00%
Contribution by the company	$ 1,336	$ 1,527	$ 304

Reportable Segments and Geogr94

Reportable Segments and Geographic Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Geographic Information
Total revenue	$ 341,599	$ 402,656	$ 384,320
Total operating loss	(134,447)	(17,637)	(41,623)
Total long-lived assets	465,618	619,116	656,629
United States
Geographic Information
Total revenue	316,756	378,497	330,003
Total operating loss	(96,228)	(8,693)	(33,067)
Total long-lived assets	465,245	547,279	582,644
Canada
Geographic Information
Total revenue	6,846	11,502	21,818
Total operating loss	(9,495)	(4,212)	(4,980)
Total long-lived assets	373	66,191	68,292
Other
Geographic Information
Total revenue	17,997	12,657	32,499
Total operating loss	(28,724)	(4,732)	(3,576)
Total long-lived assets	$ 0	$ 5,646	$ 5,693

Subsequent Events (Details)

Subsequent Events (Details) $ in Thousands	Mar. 31, 2018installment	Mar. 01, 2018USD ($)shares	Feb. 28, 2018	Feb. 27, 2018	Dec. 31, 2017USD ($)
Subsequent Event [Line Items]
Operating leases, future minimum payments due					$ 34,372
Subsequent Event
Subsequent Event [Line Items]
Operating leases, future minimum payments due		$ 8,891
Operating leases, number of monthly installments | installment	72
Subsequent Event | NG Advantage
Subsequent Event [Line Items]
Ownership interest acquired			53.50%	53.30%
Subsequent Event | NG Advantage | Common stock
Subsequent Event [Line Items]
Contractual obligation, shares issued as consideration (in shares) | shares		19,660

Schedule II_ Valuation and Qu96

Schedule II: Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowances for Doubtful Trade Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	$ 1,063	$ 1,895	$ 752
Charges (benefit) to operations	395	1,107	1,514
Deductions	(182)	(1,939)	(371)
Balance at the end of the period	1,276	1,063	1,895
Allowance for Doubtful Notes Receivables
Movement in valuation and qualifying accounts
Balance at the beginning of the period	1,230	3,990	2,850
Charges (benefit) to operations	3,344	1,617	1,142
Deductions	(30)	(4,377)	(2)
Balance at the end of the period	$ 4,544	$ 1,230	$ 3,990

Uncategorized Items - clne-2017

Label	Element	Value
Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 194,000	[1]
[1]	(1) The Company adopted two Accounting Standards Updates effective January 1, 2017 (see Note 1 for more information).