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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 03, 2018
Document and Entity Information
Entity Registrant Name	Clean Energy Fuels Corp.
Entity Central Index Key	0001368265
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)		152,534,387
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash, cash equivalents and restricted cash	$ 47,096	$ 37,208
Short-term investments	128,129	141,462
Accounts receivable, net of allowance for doubtful accounts of $1,276 and $1,353 as of December 31, 2017 and March 31, 2018, respectively	65,687	63,961
Other receivables	53,661	19,235
Inventory	37,792	35,238
Prepaid expenses and other current assets	9,425	7,793
Total current assets	341,790	304,897
Land, property and equipment, net	363,903	367,305
Notes receivable and other long-term assets, net	16,590	21,397
Investments in other entities	28,927	30,395
Goodwill	64,328	64,328
Intangible assets, net	3,217	3,590
Total assets	818,755	791,912
Current liabilities:
Current portion of debt and capital lease obligations	140,735	139,699
Accounts payable	20,266	17,901
Accrued liabilities	45,390	42,268
Deferred revenue	9,671	3,432
Total current liabilities	216,062	203,300
Long-term portion of debt and capital lease obligations	125,491	120,388
Other long-term liabilities	16,381	18,566
Total liabilities	357,934	342,254
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 151,650,969 shares and 152,514,550 shares at December 31, 2017 and March 31, 2018, respectively	15	15
Additional paid-in capital	1,113,440	1,111,432
Accumulated deficit	(672,641)	(683,570)
Accumulated other comprehensive loss	(912)	(887)
Total Clean Energy Fuels Corp. stockholders’ equity	439,902	426,990
Noncontrolling interest in subsidiary	20,919	22,668
Total stockholders’ equity	460,821	449,658
Total liabilities and stockholders’ equity	$ 818,755	$ 791,912

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,353	$ 1,276
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)	152,514,550	151,650,969
Common stock, outstanding (in shares)	152,514,550	151,650,969

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue:
Product revenue	$ 92,251	$ 76,229
Service revenue	10,152	13,262
Total revenue	102,403	89,491
Cost of sales (exclusive of depreciation and amortization shown separately below):
Product cost of sales	50,199	54,597
Service cost of sales	4,597	6,264
Selling, general and administrative	18,837	23,773
Depreciation and amortization	12,801	15,317
Total operating expenses	86,434	99,951
Operating income (loss)	15,969	(10,460)
Interest expense	(4,503)	(4,911)
Interest income	575	192
Other income (expense), net	(12)	(167)
Loss from equity method investments	(1,468)	(36)
Gain from extinguishment of debt	0	3,195
Gain from sale of certain assets of subsidiary	0	70,648
Income before income taxes	10,561	58,461
Income tax benefit (expense)	(88)	2,263
Net income	10,473	60,724
Loss attributable to noncontrolling interest	1,749	335
Net income attributable to Clean Energy Fuels Corp.	$ 12,222	$ 61,059
Income per share:
Basic (in dollars per share)	$ 0.08	$ 0.41
Diluted (in dollars per share)	$ 0.08	$ 0.40
Weighted-average common shares outstanding:
Basic (in shares)	152,194,695	148,847,503
Diluted (in shares)	156,643,092	152,972,153

Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income (loss)	$ 10,473	$ 60,724
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments, net of $0 tax in 2017 and 2018	(78)	360
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2017 and 2018	0	579
Unrealized gains (losses) on available-for-sale securities, net of $0 tax in 2017 and 2018	53	(5)
Total other comprehensive income (loss)	(25)	934
Comprehensive income (loss)	10,448	61,658
Clean Energy Fuels Corp.
Net income (loss)	12,222	61,059
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments, net of $0 tax in 2017 and 2018	(78)	360
Foreign currency adjustments on intra-entity long-term investments, net of $0 tax in 2017 and 2018	0	579
Unrealized gains (losses) on available-for-sale securities, net of $0 tax in 2017 and 2018	53	(5)
Total other comprehensive income (loss)	(25)	934
Comprehensive income (loss)	12,197	61,993
Noncontrolling Interest
Net income (loss)	(1,749)	(335)
Other comprehensive income (loss), net of tax:
Comprehensive income (loss)	$ (1,749)	$ (335)

Condensed Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Foreign currency translation adjustments, tax	$ 0	$ 0
Foreign currency adjustments on intra-entity long-term investments, tax	0	0
Unrealized (losses) gains on available-for-sale securities, tax	$ 0	$ 0

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 10,473	$ 60,724
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	12,801	15,317
Provision for doubtful accounts, notes and inventory	227	409
Stock-based compensation expense	1,898	1,910
Amortization of debt issuance cost	198	240
Gain on extinguishment of debt	0	(3,195)
Gain from sale of certain assets of subsidiary	0	(70,648)
Loss from equity method investments	1,468	36
Changes in operating assets and liabilities:
Accounts and other receivables	(36,796)	18,604
Inventory	(2,704)	162
Prepaid expenses and other assets	(1,525)	1,400
Accounts payable	3,970	(7,439)
Deferred revenue	3,914	175
Accrued expenses and other	3,727	(16,295)
Net cash provided by (used in) operating activities	(2,349)	1,400
Cash flows from investing activities:
Purchases of short-term investments	(41,723)	(30,720)
Maturities and sales of short-term investments	55,181	53,517
Purchases and deposits on property and equipment	(7,131)	(7,579)
Loans made to customers	0	(784)
Payments on and proceeds from sales of loans receivable	84	319
Cash received from sale of certain assets of subsidiary, net of cash, cash equivalents and restricted cash transferred	871	23,592
Investments in other entities	0	(1,928)
Net cash provided by investing activities	7,282	36,417
Cash flows from financing activities:
Issuances of common stock	0	10,767
Fees paid for issuances of common stock	0	(46)
Proceeds from debt instruments	6,261	6,291
Proceeds from revolving line of credit	0	0
Repayment of borrowing under revolving line of credit	0	(23,500)
Repayment of capital lease obligations and debt instruments	(1,234)	(27,250)
Net cash provided by (used in) financing activities	5,027	(33,738)
Effect of exchange rates on cash, cash equivalents and restricted cash	(72)	184
Net increase in cash, cash equivalents and restricted cash	9,888	4,263
Cash, cash equivalents and restricted cash, beginning of period	37,208	43,115
Cash, cash equivalents and restricted cash, end of period	47,096	47,378
Supplemental disclosure of cash flow information:
Income taxes paid	24	54
Interest paid, net of approximately $35 and $33 capitalized, respectively	$ 2,856	$ 3,324

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Cash Flows [Abstract]
Interest paid, capitalized	$ 33	$ 35

General	3 Months Ended
General	Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
General	General Nature of Business Clean Energy Fuels Corp., together with its majority and wholly owned subsidiaries (hereinafter collectively referred to as the "Company," unless the context or the use of the term indicates 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 Notes 3 and 4 for more information. Basis of Presentation The accompanying interim unaudited condensed consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to state fairly the Company’s financial position, results of operations, comprehensive income (loss) and cash flows as of and for the three months ended March 31, 2017 and 2018. All intercompany accounts and transactions have been eliminated in consolidation. The three month periods ended March 31, 2017 and 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any future year. Certain information and disclosures normally included in the notes to the condensed consolidated financial statements have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”), but the resultant disclosures contained herein are in accordance with accounting principles generally accepted in the United States of America (“US GAAP”) as they apply to interim reporting. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2017 that are included in the Company’s Annual Report on Form 10-K filed with the SEC on March 13, 2018. Reclassifications During the three months ended March 31, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (see Note 18). 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 condensed statement of cash flows. As a result, the Company chose to also conform this classification on the accompanying condensed balance sheets. This resulted in prior period restricted cash of $1,127 as of December 31, 2017 being reclassified into one line item with cash and cash equivalents to conform to presentation as of March 31, 2018. In addition, Deferred revenue of $175 for the three months ended March 31, 2017 was reclassified from Accrued expenses and other as a separate line item in the accompanying condensed 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. Use of Estimates The preparation of condensed consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the accompanying condensed consolidated financial statements and these 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 accompanying condensed consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived asset impairment assessments, income tax valuations and fair value measurements.

Revenue from Contracts with Customers

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Revenue from Contracts with Customers

Adoption of New Accounting Standard

On January 1, 2018, the Company adopted Revenue from Contracts with Customers (Accounting Standards Codification Topic 606) ("Topic 606" or "new guidance") retrospectively for its contracts that were not completed as of January 1, 2018, with the cumulative effective of initially applying the guidance recognized at the date of initial application ("modified retrospective method"). Results for reporting periods beginning after January 1, 2018 are presented under Topic 606, while prior period amounts are not adjusted and continue to be reported in accordance with historic accounting under Revenue Recognition (Accounting Standards Codification 605) ("Topic 605" or "previous guidance"). This adoption did not have a material impact to our condensed consolidated financial statements.

The Company recorded an increase to opening accumulated deficit of $1,293 as of January 1, 2018 due to the cumulative impact of adopting Topic 606, with the impact primarily related to the Company's volume -related revenue. As a result of applying Topic 606, during the three months ended March 31, 2018, deferred revenue and other long -term liabilities increased by $498 and notes receivable and other long -term assets, net decreased by $963, each due to the existence of significant financing components in connection with a contract for the purchase, sale and transportation of CNG and a contract for station construction, fuel and O&M services, respectively. In addition, revenue and cost of sales decreased by $119 due to certain of the Company's royalty payments being accounted for as a reduction of the transaction price and associated revenue recognized under the new guidance.

Revenue Recognition Overview

The Company recognizes revenue when control of the promised goods or services is transferred to its customers, in an amount that reflects the consideration to which it expects to be entitled in exchange for the goods or services. The Company is generally the principal in its customer contracts as it has control over the goods and services prior to them being transferred to the customer, and as such, revenue is recognized on a gross basis.

The table below presents the Company's revenues disaggregated by revenue source. Sales and usage-based taxes are excluded from revenues. Revenue is recognized net of allowances for returns and any taxes collected from customers, which are subsequently remitted to governmental authorities.



	Three Months Ended March 31,
(in thousands)	2017		2018
Volume -Related	73,574		67,219
Compressor Sales	6,467		—
Station Construction Sales	9,263		5,798
Alternative fuels excise tax credit (“AFTC”)	—		25,481
Other	187		3,905
	$	89,491	$	102,403

Performance Obligations

A performance obligation is a promise in a contract to transfer a distinct good or service to the customer, and is the unit of account in Topic 606. The performance obligations that comprise a majority of the Company's total revenue consist of (1) construction of and sale of a station or modification/upgrade of an existing station, (2) providing O&M services for the station, and (3) sale of fuel to a customer. In certain contracts with customers, the Company agrees to provide multiple goods or services, which include various combinations of these three performance obligations. These contracts have multiple performance obligations because the promise to transfer each separate good or service is separately identifiable from other promises in the contracts and, therefore, each is distinct. This evaluation requires significant judgment and the decision to combine a group of contracts or separate the combined or single contract into multiple performance obligations could change the amount of revenue recognized in one or more periods. The Company allocates a contract’s transaction price to each performance obligation using best estimates of the standalone selling price of each distinct good or service in the contract. The primary method used to estimate the standalone selling price for fuel and O&M services is observable standalone sales, and the primary method used to estimate the standalone selling price for station construction sales is the expected cost plus a margin approach, because the Company sells customized customer -specific solutions. Under this approach, the Company forecasts expected costs of satisfying a performance obligation and then adds an appropriate margin for the good or service.

Nature of Goods and Services

Volume -Related

The Company’s volume -related revenue primarily consists of sales of CNG, LNG and RNG fuel, RINs and LCFS Credits and O&M services. Fuel and O&M services are sold pursuant to contractual commitments over defined goods -and -service delivery periods. These contracts typically include a stand -ready obligation to supply natural gas and/or provide O&M services daily based on a committed and agreed upon routine maintenance schedule or when and if called upon by the customer.

The Company recognizes revenue over time for fuel sales and O&M service sales because the customer receives and consumes the benefits provided by the Company's performance as the stand -ready obligations are being performed.

The Company seeks to sell RINs and LCFS Credits (the "government credits") to third parties who need the credits to comply with federal and state requirements. The government credits are considered variable consideration because they can either increase or decrease the transaction price based on volumes of vehicle fuel sold. Additionally, these government credits are constrained until there is an agreement in place to monetize the credits at a determinable price, at which time the constraint is removed and the government credits are included in the transaction price and revenue is recognized. RINs and LCFS Credits are included in volume -related revenues.

Payment terms and conditions vary by contract type. For substantially all the Company's of contracts under which it receives volume -related revenue, the timing of revenue recognition does not differ from the timing of invoicing; as a result the Company has determined these contracts generally do not include a significant financing component.

Compressor Sales

Because the Company completed the CEC Combination during the year ended December 31, 2017 and Topic 606 was adopted effective January 1, 2018, Topic 606 is not applicable to this source of revenue.

Station Construction Sales

Station construction contracts are generally short-term, except for certain larger and more complex stations, which can take up to 24 months to complete. For most of the Company's station construction contracts, the customer contracts with the Company to provide a significant service of integrating a complex set of tasks and components into a single station. Hence, the entire contract is accounted for as one performance obligation. Also, as discussed under Performance Obligations above, certain of the Company's station construction contracts include other distinct goods or services, which requires the contract to be separated into more than one performance obligation.

The Company generally recognizes revenue over time as the Company performs under its station construction contracts because of the continuous transfer of control of the goods to the customer, who typically controls the work in process. Revenue is recognized based on the extent of progress towards completion of the performance obligation and is recorded proportionally as costs are incurred. Costs to fulfill the Company's obligations under these contracts typically include labor, materials and subcontractors’ costs, other direct costs and an allocation of indirect costs.

Under the typical payment terms of the Company's station construction contracts, the customer makes either performance-based payments ("PBPs") or progress payments. PBPs are interim payments of the contract price based on quantifiable measures of performance or the achievement of specified events or milestones. Progress payments are interim payments of costs incurred as the work progresses. For some of these contracts, the Company may be entitled to receive an advance payment which is recognized as a liability because payment is in excess of revenue recognized and is presented as contract liabilities on the consolidated balance sheet. The advance payment typically is not considered a significant financing component because it is used to meet working capital demands that can be higher in the early stages of a construction contract and to protect the Company if the counter -party fails to adequately complete some or all of its obligations under the contract. In addition, because the customer retains a small portion of the contract price until completion of the contract, these contracts can also result in revenue recognized in excess of billings which the Company presents as contract assets on its consolidated balance sheet. Amounts billed and due from customers are classified as receivables on the Company's consolidated balance sheet. The portion of the payments retained by the customer until final contract settlement is not considered a significant financing component because the intent is to protect the customer.

AFTC

See Volume -Related Revenue for a description on how the Company recognizes revenue on the government credits, which is similar for AFTC and see Note 17 for more information about AFTC generally.

Other

The majority of this other revenue is from sales of used natural gas heavy -duty trucks purchased by the Company. Revenue on these contracts is recognized at a point in time when the customer accepts delivery of the truck.

Significant Judgments and Management's Estimates Related to Revenue Recognition

Due to the nature of the work required to be performed under contracts that combine multiple performance obligations, the estimation of total revenue and cost at completion is subject to variables and requires significant judgment. As previously mentioned in Volume -Related above, the government credits are provisions that can either increase or decrease the transaction price based on the volume of vehicle fuel sold. The Company estimates variable consideration as the most likely amount to which it expects to be entitled. The Company includes estimated amounts in the transaction price to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is resolved. The Company's estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are largely based on an assessment of the Company's anticipated performance and all other information (historical, current and forecasted) that is reasonably available.

The Company's contract modifications are primarily for goods or services that are not distinct from the existing contract and are typically renewals of fuel and O&M service sales or expansions in scope of an existing station construction. As a result, these modifications are accounted for as if they were part of the existing contract. The effect of a contract modification on the transaction price and the Company's measure of progress for the performance obligation to which it relates, is recognized as an adjustment to revenue (either as an increase or a reduction) on a cumulative catch-up basis for station construction contracts and prospectively for fuel and O&M service sale contracts.

With respect to the Company's station construction contracts, 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 the contract, including differing site conditions, the availability of skilled contract labor, the performance of major suppliers and subcontractors, and unexpected changes in material costs. Because a significant change in one or more of these estimates could affect the profitability of these contracts, the contract price and cost estimates are reviewed periodically as work progresses and adjustments proportionate to the cost-to-cost measure of progress are reflected in contract revenues in the reporting period when such estimates are revised as discussed above. Provisions for estimated losses on uncompleted contracts are made in the period in which the losses become known. During the three months ended March 31, 2018, there were no significant losses on open contracts, significant contract penalties, settlements or changes in contract estimates.

Remaining Performance Obligations

Remaining performance obligations represents the transaction price of customer orders for which work has not been performed. As of March 31, 2018, the aggregate amount of the transaction price allocated to remaining performance obligations was $11,562, which related to the Company's station construction sale contracts. The Company expects to recognize revenue on the remaining performance obligations under these contracts over the next 12 to 24 months.

The Company has elected to apply an optional exemption under Topic 606 for its volume -related revenue, which waives the requirement to disclose the remaining performance obligation for revenue recognized from the satisfaction of the performance obligation through the "right to invoice" practical expedient. The nature of the performance obligations are the stand ready obligations to supply natural gas and/or provide O&M services daily. These performance obligations are variable consideration and constrained because the Company bills dependent upon the amount gasoline gallon equivalents of natural gas dispensed and current pricing conditions. The transaction price to allocate to the performance obligations is known and the constraint is removed upon monthly billing to the customer.

Costs to Fulfill a Contract

The Company capitalizes costs incurred to fulfill its contracts that (1) relate directly to the contract, (2) are expected to generate resources that will be used to satisfy the Company’s performance obligations under the contract, and (3) are expected to be recovered through revenue generated under the contract. Contract fulfillment costs are recorded to depreciation expense as the Company satisfies its performance obligations over the term of the contract. These costs primarily relate to set-up and other direct installation costs incurred by our subsidiary, NG Advantage, LLC ("NG Advantage") for equipment that must be installed on customer land before NG Advantage is able to deliver CNG to the customer, because the customer does not have direct access to the natural gas pipelines. These costs are classified in land, property, and equipment, net in the accompanying condensed consolidated balance sheets. As of March 31, 2018, these costs incurred to fulfill contracts were $7,123 with accumulated depreciation of $3,888 and related amortization of $511 for the three months ended March 31, 2018.

Contract Balances

The timing of revenue recognition, billings and cash collections results in billed accounts receivable, unbilled receivables (contract assets), and customer advances and deposits (contract liabilities) in the accompanying condensed consolidated balance sheets. Changes in the contract asset and liability balances during the three months ended March 31, 2018, were not materially impacted by any other factors outside of normal course of business.

Receivables, Net

Receivables, net, include amounts billed and currently due from customers. The amounts due are stated at their net estimated realizable value. The Company maintains an allowance for doubtful accounts to provide for the estimated amount of receivables that will not be collected. The allowance is based upon an assessment of customer creditworthiness, historical payment experience, and the age of outstanding receivables. Receivables, net were $63,961 and $65,687 as of December 31, 2017 and March 31, 2018, respectively.

Contract Assets

Contract assets include unbilled amounts typically resulting from the Company's station construction sale contracts, when the cost-to-cost method of revenue recognition is utilized and revenue recognized exceeds the amount billed to the customer, and right to payment is not just subject to the passage of time. Amounts may not exceed their net realizable value. Contract assets of $1,356 and $1,249 are classified as current and included in prepaid expenses and other current assets in the accompanying condensed consolidated balance sheets as of December 31, 2017 and March 31, 2018, respectively.

Contract Liabilities

Contract liabilities consist of billings in excess of revenue recognized from the Company's station construction sale contracts and deferred revenue when cash payments are received or due in advance of the Company's performance obligation which are generally for the Company's volume -related revenue contracts. Billings in excess of revenue recognized of $1,092 and $4,110 are classified as current and are included in deferred revenue in the accompanying condensed consolidated balance sheets as of December 31, 2017 and March 31, 2018, respectively. Deferred revenue is classified as current or noncurrent based on when the revenue is expected to be recognized. The current portion of deferred revenue was $3,432 and $9,671 as of December 31, 2017 and March 31, 2018, respectively, and the noncurrent portion of deferred revenue of $13,413 and $11,412 is included in other long -term liabilities in the accompanying condensed consolidated balance sheets as of December 31, 2017 and March 31, 2018, respectively.

Revenue recognized during the three months ended March 31, 2018 related to the Company's contract liability balances as of December 31, 2017 was $1,842.

Divestitures	3 Months Ended
Divestitures	Mar. 31, 2018
Discontinued Operations and Disposal Groups [Abstract]
Divestitures	Divestitures 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 RNG production facilities, Renewables’ interest in joint ventures formed with a third party to develop new RNG production facilities, 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 12), 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, which was paid in full on April 3, 2017. In addition, as a result of the determination of certain post-closing adjustments, (i) BP paid Renewables an additional $2,010 on June 22, 2017, and (ii) the gain recorded from the BP Transaction was reduced by $762 subsequent to March 31, 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 accompanying condensed consolidated statement of operations for 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 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, net as of December 31, 2017, which is included in the total gain on the BP Transaction. The Company incurred $3,695 in transaction fees in connection with the BP Transaction, and subsequent to March 31, 2017 through March 31, 2018, the Company has paid $8,605 in cash and issued 770,269 shares of the Company's common stock, collectively valued at $1,964, to former holders of options to purchase membership units in Renewables (the "Option Holders"). The net proceeds as of March 31, 2018 from the BP Transaction, net of $1,007 cash transferred to BP, were $143,061. 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 $70,648 as of December 31, 2017. Included in the amount of total gain 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 significant disposal nor was the disposal a strategic shift in the Company's strategy.

Investments in Other Entities and Noncontrolling Interest in a Subsidiary	3 Months Ended
	Mar. 31, 2018
Business Combinations [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 November 26, 2017, the Company, through its former subsidiary, Clean Energy Compression Corp. ("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 recorded a loss from this investment of $1,441 for the three months ended March 31, 2018. The Company has an investment balance in SAFE&CEC S.r.l. of $27,883 and $26,442 as of December 31, 2017 and March 31, 2018, 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. 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, because the Company does not control but has the ability to exercise significant influence over MCEP’s operations. The Company recorded a loss from this investment of $36 and $27 for the three months ended March 31, 2017 and 2018, respectively. The Company has an investment balance in MCEP of $1,512 and $1,485 as of December 31, 2017 and March 31, 2018, respectively. NG Advantage On October 14, 2014, the Company entered into a Common Unit Purchase Agreement (“UPA”) with NG Advantage for a 53.3% controlling interest in NG Advantage. NG Advantage is engaged in the business of transporting CNG in high-capacity trailers to industrial and institutional energy users, such as hospitals, food processors, manufacturers and paper mills that do not have direct access to natural gas pipelines. The Company viewed the acquisition as a strategic investment in the expansion of the Company’s initiative to deliver natural gas to industrial and institutional energy users. 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%. On February 28, 2018, the Company entered into a guaranty agreement with NG Advantage and one of its customers for the purchase, sale and transportation of CNG. The Company guarantees NG Advantage's payment obligations in the event of default up to $30,000 plus related fees. This guaranty is in effect until thirty days following the Company's notice to NG Advantage's customer of its termination. As consideration for the guaranty agreement, NG Advantage issued to the Company 19,660 common units, which increased the Company's controlling interest in NG Advantage from 53.3% to 53.5%. Net income included a loss from the noncontrolling interest in NG Advantage of $335 and $1,749 for the three months ended March 31, 2017 and 2018, respectively. The noncontrolling interest was $22,668 and $20,919 as of December 31, 2017 and March 31, 2018, respectively.

Cash, Cash Equivalents, and Restricted Cash	3 Months Ended
Cash, Cash Equivalents, and Restricted Cash	Mar. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash, Cash Equivalents, and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments with maturities of three months or less on the date of acquisition to be cash equivalents. The Company places its cash and cash equivalents with high credit quality financial institutions. At times, such investments may be in excess of the Federal Deposit Insurance Corporation (“FDIC”) and Canadian Deposit Insurance Corporation (“CDIC”). Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash deposits. The amounts in excess of FDIC and other foreign insurance limits were approximately $34,709 and $43,793 as of December 31, 2017 and March 31, 2018, respectively. 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, 2017 and March 31, 2018 consisted of standby letters of credit renewed annually in an aggregate amount of $1,127, and an additional $750 held in escrow as of March 31, 2018. As of December 31, 2017 and March 31, 2018, the Company had no long-term restricted cash.

Investments

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Investments

Available-for-sale securities are carried at fair value, inclusive of unrealized gains and losses. During the three months ended March 31, 2018, the Company adopted ASU No. 2016-01, Financial Instruments: Recognition and Measurement of Financial Assets and Financial Liabilities (see Note 18 for more information). Upon adoption, unrealized gains and losses are included in other income (expense), net for equity securities. Unrealized gains and losses for debt securities continue to be recognized 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.

Short-term investments as of December 31, 2017 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
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

Short-term investments as of March 31, 2018 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds and notes	$	21,481	$	(116	)	$	21,365
Zero coupon bonds	52,153		(34		)	52,119
Corporate bonds	43,737		(37		)	43,700
Certificate of deposits	10,945		—			10,945
Total short-term investments	$	128,316	$	(187	)	$	128,129

Fair Value Measurements

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

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

As of March 31, 2018, 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, 2017 and March 31, 2018. See Note 12 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, 2017 and March 31, 2018, 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, 2017 and March 31, 2018, respectively:



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



Description	Balance at March 31, 2018		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	21,365	$	—	$	21,365	$	—
Zero coupon bonds	52,119		—		52,119		—
Corporate bonds	43,700		—		43,700		—
Certificate of deposits	10,945		—		10,945		—
Liabilities:
Warrants(2)	515		—		—		515

(1) Included in short-term investments in the accompanying condensed consolidated balance sheets. See Note 6 for more information.

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

- Definition

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

+ References + Details

Other Receivables

3 Months Ended

Mar. 31, 2018

Receivables [Abstract]

Other Receivables

Other receivables as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Loans to customers to finance vehicle purchases	$	4,746	$	5,163
Accrued customer billings	10,072		6,465
Fuel tax credits	177		36,935
Other	4,240		5,098
Total other receivables	$	19,235	$	53,661

- Definition

The entire disclosure for claims held for amounts due a entity, excluding financing receivables. Examples include, but are not limited to, trade accounts receivables, notes receivables, loans receivables. Includes disclosure for allowance for credit losses.

+ References + Details

Inventory

3 Months Ended

Mar. 31, 2018

Inventory Disclosure [Abstract]

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 as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Raw materials and spare parts	$	35,145	$	37,702
Finished goods	93		90
Total inventories	$	35,238	$	37,792

Land, Property and Equipment

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Land, Property and Equipment

Land, property and equipment as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017			March 31, 2018
Land	$	2,858		$	2,858
LNG liquefaction plants	94,634			94,634
Station equipment	304,090			306,666
Trailers	70,906			71,565
Other equipment	88,313			93,555
Construction in progress	74,905			74,748
	635,706			644,026
Less: accumulated depreciation	(268,401		)	(280,123		)
Total land, property and equipment, net	$	367,305		$	363,903

Included in land, property and equipment are capitalized software costs of $26,003 and $27,379 as of December 31, 2017 and March 31, 2018, respectively. The accumulated amortization of the capitalized software costs is $18,737 and $19,455 as of December 31, 2017 and March 31, 2018, respectively.

The Company recorded amortization expense related to the capitalized software costs of $872 and $718 during the three months ended March 31, 2017 and 2018, respectively.

As of March 31, 2017 and 2018, $2,884 and $939, respectively, are included in accounts payable and accrued liabilities balances, which amounts are related to purchases of property and equipment. These amounts are excluded from the condensed consolidated statements of cash flows as they are non-cash investing activities.

Accrued Liabilities

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Accrued Liabilities

Accrued liabilities as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Accrued alternative fuels incentives (1)	$	2,954	$	14,812
Accrued employee benefits	2,378		2,983
Accrued interest	1,486		35
Accrued gas and equipment purchases	8,722		7,630
Accrued property and other taxes	4,582		5,641
Salaries and wages	8,363		2,787
Other (2)	13,783		11,502
Total accrued liabilities	$	42,268	$	45,390


(1)	Includes the amount of RINs and LCFS Credits and, as of March 31, 2018, the amount of AFTC payable to third parties. The AFTC had expired as of December 31, 2017, but was reinstated in February 2018 for vehicle fuel sales made from January 1, 2017 through December 31, 2017. See Note 17 for more information about AFTC.


(2)	The amount as of December 31, 2017 and March 31, 2018 includes lease termination fees and asset retirement obligations related to the closure of certain fueling stations and working capital adjustments, in the third and fourth quarters of 2017, funding for certain commitments, and transaction fees incurred as a result of the CEC Combination (see Note 4 for more information).

Debt

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Debt

Debt and capital lease obligations as of December 31, 2017 and March 31, 2018 consisted of the following and are further discussed below:



	December 31, 2017
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes	$	125,000		$	131		$	124,869
5.25% Notes	110,450			454			109,996
Capital lease obligations	802			—			802
NG Advantage debt	23,437			259			23,178
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



	March 31, 2018
	Principal Balances			Unamortized Debt Financing Costs			Balance Net of Financing Costs
7.5% Notes	$	125,000		$	108		$	124,892
5.25% Notes	110,450			301			110,149
Capital lease obligations	882			—			882
NG Advantage debt	29,437			322			29,115
Other debt	1,188			—			1,188
Total debt and capital lease obligations	266,957			731			266,226
Less amounts due within one year	(141,106		)	(371		)	(140,735		)
Total long-term debt and capital lease obligations	$	125,851		$	360		$	125,491

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”), our co-founder and board member T. Boone 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 SEC.

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 March 31, 2018.

On August 27, 2013, GEIH transferred $5,000 in principal amount of its 7.5% Notes to certain 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 three months ended March 31, 2017.

On February 21, 2017, GEIH transferred $11,800 in principal amount of its 7.5% Notes to certain 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 March 31, 2018, (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.

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 March 31, 2018.

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.

The Company has paid an aggregate of $84,344 in cash and issued an aggregate of 6,265,829 shares of its common stock to repurchase or exchange an aggregate of $139,550 in aggregate principal amount of the 5.25% Notes, together with all accrued and unpaid interest thereon. No such repurchases or exchanges occurred during the three months ended March 31, 2017 or 2018. 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.

Plains Credit Facility

On February 29, 2016, the Company entered into a Loan and Security Agreement (the “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 March 31, 2018.

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 March 31, 2018.

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

NG Advantage has debt for trailers and equipment due at various dates through 2025 bearing interest at rates up to 8.76%, with weighted -average interest rates of 4.98% and 5.88%, as of December 31, 2017 and March 31, 2018, respectively. NG Advantage pledged to and granted a security interest in all of its right, title and interest in the CNG trailers and equipment purchased with the proceeds received from various creditors.

Other Debt

The Company has other debt due at various dates through 2023 bearing interest at rates up to 5.02%, with weighted -average interest rates of 4.79% and 4.79% as of December 31, 2017 and March 31, 2018, respectively.

Net Income Per Share

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Income Per Share

Basic net income per share is computed by dividing the net income 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 income per share is computed by dividing the net income 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 income per share if their effect would be antidilutive.

The information required to compute basic and diluted net income per share is as follows:



	Three Months Ended March 31,
	2017	2018
Weighted-average common shares outstanding	148,847,503	152,194,695
Dilutive effect of potential common shares from restricted stock units and stock options	4,124,650	4,448,397
Weighted-average common shares outstanding - diluted	152,972,153	156,643,092

The following potentially dilutive securities have been excluded from the diluted net income per share calculations because their effect would have been antidilutive. Although these securities were antidilutive for these periods, they could be dilutive in future periods.



	Three Months Ended March 31,
	2017	2018
Stock Options	12,426,603	8,573,749
Convertible Notes	14,991,521	14,991,521
Total	27,418,124	23,565,270

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 period presented:



	Three Months Ended March 31,
(in 000s, except per-share amounts)	2017
Gross proceeds	$	10,767
Fees and issuance costs	254
Net proceeds	$	10,513
Shares issued	3,802,500

Stock-Based Compensation

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

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 accompanying condensed consolidated statements of operations during the periods:



	Three Months Ended March 31,
	2017		2018
Stock-based compensation expense, net of $0 tax in 2017 and 2018	$	1,910	$	1,898

As of March 31, 2018, there was $8,340 of total unrecognized compensation costs related to unvested shares subject to outstanding stock options and restricted stock units, which is expected to be expensed over a weighted-average period of approximately 2.34 years.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company’s income tax benefit (expense) was $2,263 and $(88) for the three months ended March 31, 2017 and 2018, respectively. Tax benefit (expense) for all periods was comprised of taxes due on the Company’s U.S. and foreign operations. The increase in the Company’s income tax expense for the three months ended March 31, 2018 as compared to the tax benefit for the three months ended March 31, 2017 was primarily due to a decrease in the deferred tax benefit attributed to the reduction of goodwill amortization following the BP Transaction (see Note 3). The effective tax rates for the three months ended March 31, 2017 and 2018 are different from the federal statutory tax rate primarily as a result of losses for which no tax benefit has been recognized. Following the BP Transaction, the Company also benefited from the utilization of federal and state net operating loss ("NOL") carryovers that offset all of the Company's federal and the majority of its state taxes. In addition to the decrease in its deferred tax liability of $2,493 attributed to the reduction in goodwill following the BP Transaction, the utilization of NOLs also resulted in a decrease of $29,768 in the Company's deferred tax assets attributed to NOLs and a corresponding decrease in the Company's deferred tax asset valuation allowance. The Company increased its liability for unrecognized tax benefits in the three months ended March 31, 2018 by $2,689, which was attributable to the portion of AFTC revenue recognized in the period that was offset by the fuel tax the Company collected from its customers as an unrecognized tax benefit during the year ended December 31, 2017. The net interest incurred was immaterial for both the three months ended March 31, 2017 and 2018, respectively.

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2018
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.

Alternative Fuels Excise Tax Credit	3 Months Ended
Alternative Fuels Excise Tax Credit	Mar. 31, 2018
Alternative Fuels Excise Tax Credit
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 the AFTC tax credit 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 most recent legislation authorizing AFTC being signed into law on February 9, 2018, all AFTC revenue for vehicle fuel the Company sold in the 2017 calendar year, has been recognized in the three months ended March 31, 2018 and will be received subsequent that date. AFTC revenue recognized for the three months ended March 31, 2017 and 2018 was $0 and $25,481, respectively. AFTC is not currently available, and may not be reinstated, for vehicle fuel sales made after December 31, 2017.

Recently Adopted Accounting Changes and Recently Issued Accounting Standards	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recently Adopted Accounting Changes and Recently Issued Accounting Standards	Recently Adopted Accounting Changes and Recently Issued Accounting Standards Recently Adopted Accounting Changes In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects From Accumulated Other Comprehensive Income, which allows for a reclassification from accumulated other comprehensive income (AOCI) to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (the "TCJA"). This update is effective for fiscal years beginning after December 15, 2018, which for the Company is the first quarter of 2019, with early adoption permitted. The Company elected to early adopt this ASU during the three months ended March 31, 2018, which did not have any impact on the accompanying condensed consolidated financial statements or related disclosures. The Company did not elect to reclassify the stranded tax effects of the TCJA as there were none. 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. The Company adopted this standard on a retrospective basis, and adoption did not have a material impact on the Company’s consolidated financial statements or related disclosures. As a result of including restricted cash with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts presented in the accompanying condensed consolidated statement of cash flows, net cash flows decreased by $6,743 for the three months ended March 31, 2017 (see Note 1). 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, which for the Company is the first quarter of 2018. Adoption of this standard did not have any impact on the accompanying condensed consolidated financial statements and related disclosures for the three months ended March 31, 2018. In January 2016, the FASB issued ASU 2016-01, Financial Instruments: Recognition and Measurement of Financial Assets and Financial Liabilities. In February 2018, the FASB subsequently issued ASU 2018-03, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): 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. Adoption of this standard did not have any impact on the accompanying consolidated financial statements and related disclosures for the three months ended March 31, 2018. 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 Company adopted this standard using the modified retrospective method. See Note 2 of these condensed consolidated financial statements for more information and related disclosures. Recently Issued Accounting Standards In February 2016, the FASB issued ASU 2016-02, Leases and in January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842. 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 adoption using a modified retrospective method. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On May 9, 2018, the Company entered into a stock purchase agreement (the “Purchase Agreement”) with Total Marketing Services, S.A., a wholly owned subsidiary of Total S.A. (“Total”). Pursuant to the Purchase Agreement, the Company agreed to sell and issue, and Total agreed to purchase, up to 50,856,296 shares of the Company's common stock at a purchase price of $1.64 per share, all in a private placement (the “Total Private Placement”). The purchase price per share was determined based on the volume-weighted average price for the Company’s common stock between March 23, 2018 (the day on which discussions began between the Company and Total) and May 3, 2018 (the day on which the Company agreed in principle with Total regarding the structure and basic terms of its investment). If all of the shares to be sold under the Purchase Agreement are issued, then the Company would receive gross proceeds from such sale of $83,404 and, immediately after such issuance (and based on the number of shares of the Company’s common stock outstanding as of April 10, 2018, which was 152,514,550), Total would hold 25.0% of the outstanding shares of the Company's common stock and the largest ownership position of the Company. As of the date of the Purchase Agreement, Total did not hold or otherwise beneficially own any shares of the Company’s common stock, and Total has agreed, until the later of May 9, 2020 or such date when it ceases to hold more than 5.0% of the Company’s common stock then outstanding, among other similar undertakings and subject to customary conditions and exceptions, to not purchase shares of the Company’s common stock or otherwise pursue transactions that would result in Total beneficially owning more than 30.0% of the Company’s equity securities without the approval of the Company’s board of directors. Pursuant to the Purchase Agreement, the completion of the Total Private Placement is conditioned on the satisfaction or waiver (if and to the extent permitted by applicable laws, rules and regulations) of certain specified conditions, including, among others, that the Company obtains the approval of its stockholders at its 2018 annual stockholders’ meeting of the issuance of all of the shares to be sold under the Purchase Agreement (as and to the extent required by applicable rules of the Nasdaq Stock Market) and the amendment of the Company’s Restated Certificate of Incorporation to increase the number of shares of its common stock it is authorized to issue thereunder. Pursuant to the Purchase Agreement, if the Company fails to obtain these approvals, then Total would have the right, exercisable in its sole discretion within two calendar weeks after the conclusion of the Company’s 2018 annual stockholders’ meeting, to elect to purchase fewer shares of the Company’s common stock, in an amount equal to 19.99% of the lesser of the number of shares of the Company’s common stock outstanding immediately before the Purchase Agreement was signed (which was 152,568,887 shares and 19.99% of which is 30,498,520 shares), and the number of shares of the Company’s common stock outstanding immediately before Total’s delivery to the Company of a notice indicating its election to exercise such right. Any such sale, issuance and purchase of fewer shares of the Company’s common stock would be completed under the terms of the Purchase Agreement, including the price per share set forth above, and would result in gross proceeds to the Company of up to $50,018. The Purchase Agreement also provides that Total will have the right to designate up to two individuals to serve as directors on the Company’s board of directors. Subject to certain limited conditions as described in the Purchase Agreement, including compliance with the Company’s governing documents and all applicable laws, rules and regulations, the Company will be obligated to appoint or nominate for election as directors of the Company the individuals so designated by Total and, from and after such appointment or election, appoint one of these individuals to serve on the audit committee of the Company’s board of directors and any other board committees that may be formed from time to time for the purpose of making decisions that are strategically significant to the Company. Total’s rights and our obligations relating to these designees commence at the time any shares are issued to Total under the Purchase Agreement, and continue until (and if) (1) with respect to Total’s right to designate two individuals to serve as directors on the Company’s board of directors, Total’s voting power is less than 16.7% but more than 10.0%, and (2) with respect to Total’s right to designate one individual to serve as a director on the Company’s board of directors, Total’s voting power is less than 10.0%, in each case measured in relation to the total votes then entitled to be cast in an election of directors by the Company’s stockholders. The Purchase Agreement also contains representations, warranties and other covenants made by the Company and Total that are customary for transactions of this nature. The Company expects the Total Private Placement to be completed promptly following the satisfaction of all conditions as set forth in the Purchase Agreement. The Company expects to use any net proceeds received from the Total Private Placement for working capital and general corporate purposes, which may include, among other purposes, executing its business plans, pursuing opportunities for further growth, and retiring a portion of its outstanding indebtedness. Pursuant to the Purchase Agreement, the Company has also agreed to enter into a registration rights agreement with Total at the closing of the issuance and sale of its common stock to Total under the Purchase Agreement. Pursuant to the registration rights agreement, the Company will be obligated to, at its expense, (1) within 60 days after the issuance and sale of shares of its common stock to Total under the Purchase Agreement, file one or more registration statements with the SEC to cover the resale of such shares, (2) use its commercially reasonable efforts to cause all such registration statements to be declared effective within 90 days after the initial filing thereof with the SEC, (3) use its commercially reasonable efforts to maintain the effectiveness of such registration statements until all such shares are sold or may be sold without restriction under Rule 144 under the Securities Act of 1933, as amended, and (4) with a view to making available to the holders of such shares the benefits of Rule 144, make and keep available adequate current public information, as defined in Rule 144, and timely file with the SEC all required reports and other documents, until all such shares are sold or may be sold without restriction under Rule 144. If such registration statements are not filed or declared effective as described above or any such effective registration statements subsequently become unavailable for more than 30 days in any 12-month period while they are required to maintained as effective, then the Company would be required to pay liquidated damages to Total equal to 0.75% of the aggregate purchase price for the shares remaining eligible for such registration rights each month for each such failure (up to a maximum of 4.0% of the aggregate purchase price for the shares remaining eligible for such registration rights each year). In addition, and separate from the Total Private Placement, the Company has entered into a non-binding letter of intent with Total, in which both parties have agreed to negotiate in good faith regarding the launch of a truck leasing program and related credit support arrangement designed to facilitate and grow the deployment of heavy-duty natural gas trucks in the United States. This program and arrangement are subject to completion of definitive agreements, and as a result, may not be launched when or as expected, on terms similar to those contemplated by the letter of intent, or at all.

General (Policies)

3 Months Ended

Mar. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of Presentation

The accompanying interim unaudited condensed consolidated financial statements include the accounts of the Company and its subsidiaries, and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to state fairly the Company’s financial position, results of operations, comprehensive income (loss) and cash flows as of and for the three months ended March 31, 2017 and 2018. All intercompany accounts and transactions have been eliminated in consolidation. The three month periods ended March 31, 2017 and 2018 are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or for any other interim period or for any future year.

Certain information and disclosures normally included in the notes to the condensed consolidated financial statements have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”), but the resultant disclosures contained herein are in accordance with accounting principles generally accepted in the United States of America (“US GAAP”) as they apply to interim reporting. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2017 that are included in the Company’s Annual Report on Form 10-K filed with the SEC on March 13, 2018.

Reclassifications

During the three months ended March 31, 2018, the Company adopted Accounting Standards Update ("ASU") No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (see Note 18). 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 condensed statement of cash flows. As a result, the Company chose to also conform this classification on the accompanying condensed balance sheets. This resulted in prior period restricted cash of $1,127 as of December 31, 2017 being reclassified into one line item with cash and cash equivalents to conform to presentation as of March 31, 2018. In addition, Deferred revenue of $175 for the three months ended March 31, 2017 was reclassified from Accrued expenses and other as a separate line item in the accompanying condensed 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.

Use of Estimates

The preparation of condensed consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the amounts reported in the accompanying condensed consolidated financial statements and these 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 accompanying condensed consolidated financial statements include (but are not limited to) those related to revenue recognition, goodwill and long-lived asset impairment assessments, income tax valuations and fair value measurements.

Recently Adopted Accounting Changes and Recently Issued Accounting Standards

Adoption of New Accounting Standard

Recently Adopted Accounting Changes

In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects From Accumulated Other Comprehensive Income, which allows for a reclassification from accumulated other comprehensive income (AOCI) to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (the "TCJA"). This update is effective for fiscal years beginning after December 15, 2018, which for the Company is the first quarter of 2019, with early adoption permitted. The Company elected to early adopt this ASU during the three months ended March 31, 2018, which did not have any impact on the accompanying condensed consolidated financial statements or related disclosures. The Company did not elect to reclassify the stranded tax effects of the TCJA as there were none.

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. The Company adopted this standard on a retrospective basis, and adoption did not have a material impact on the Company’s consolidated financial statements or related disclosures. As a result of including restricted cash with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts presented in the accompanying condensed consolidated statement of cash flows, net cash flows decreased by $6,743 for the three months ended March 31, 2017 (see Note 1).

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, which for the Company is the first quarter of 2018. Adoption of this standard did not have any impact on the accompanying condensed consolidated financial statements and related disclosures for the three months ended March 31, 2018.

In January 2016, the FASB issued ASU 2016-01, Financial Instruments: Recognition and Measurement of Financial Assets and Financial Liabilities. In February 2018, the FASB subsequently issued ASU 2018-03, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): 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. Adoption of this standard did not have any impact on the accompanying consolidated financial statements and related disclosures for the three months ended March 31, 2018.

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 Company adopted this standard using the modified retrospective method. See Note 2 of these condensed consolidated financial statements for more information and related disclosures.

Recently Issued Accounting Standards

In February 2016, the FASB issued ASU 2016-02, Leases and in January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842. 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 adoption using a modified retrospective method. The Company is evaluating the impact this ASU will have on its consolidated financial statements and related disclosures.

Revenue Recognition

Revenue Recognition Overview



	Three Months Ended March 31,
(in thousands)	2017		2018
Volume -Related	73,574		67,219
Compressor Sales	6,467		—
Station Construction Sales	9,263		5,798
Alternative fuels excise tax credit (“AFTC”)	—		25,481
Other	187		3,905
	$	89,491	$	102,403

Performance Obligations

Nature of Goods and Services

Volume -Related

Compressor Sales

Because the Company completed the CEC Combination during the year ended December 31, 2017 and Topic 606 was adopted effective January 1, 2018, Topic 606 is not applicable to this source of revenue.

Station Construction Sales

AFTC

See Volume -Related Revenue for a description on how the Company recognizes revenue on the government credits, which is similar for AFTC and see Note 17 for more information about AFTC generally.

Other

Significant Judgments and Management's Estimates Related to Revenue Recognition

Remaining Performance Obligations

Costs to Fulfill a Contract

Contract Balances

Receivables, Net

Contract Assets

Contract Liabilities

Revenue recognized during the three months ended March 31, 2018 related to the Company's contract liability balances as of December 31, 2017 was $1,842

Revenue from Contracts with Customers (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue



	Three Months Ended March 31,
(in thousands)	2017		2018
Volume -Related	73,574		67,219
Compressor Sales	6,467		—
Station Construction Sales	9,263		5,798
Alternative fuels excise tax credit (“AFTC”)	—		25,481
Other	187		3,905
	$	89,491	$	102,403

Investments (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Summary of short-term investments

Short-term investments as of December 31, 2017 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
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

Short-term investments as of March 31, 2018 consisted of the following:



	Amortized Cost		Gross Unrealized Losses			Estimated Fair Value
Municipal bonds and notes	$	21,481	$	(116	)	$	21,365
Zero coupon bonds	52,153		(34		)	52,119
Corporate bonds	43,737		(37		)	43,700
Certificate of deposits	10,945		—			10,945
Total short-term investments	$	128,316	$	(187	)	$	128,129

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

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, 2017 and March 31, 2018, respectively:



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



Description	Balance at March 31, 2018		Level 1		Level 2		Level 3
Assets:
Available-for-sale securities(1):
Municipal bonds and notes	$	21,365	$	—	$	21,365	$	—
Zero coupon bonds	52,119		—		52,119		—
Corporate bonds	43,700		—		43,700		—
Certificate of deposits	10,945		—		10,945		—
Liabilities:
Warrants(2)	515		—		—		515

(1) Included in short-term investments in the accompanying condensed consolidated balance sheets. See Note 6 for more information.

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

Other Receivables (Tables)

3 Months Ended

Mar. 31, 2018

Receivables [Abstract]

Schedule of other receivables

Other receivables as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Loans to customers to finance vehicle purchases	$	4,746	$	5,163
Accrued customer billings	10,072		6,465
Fuel tax credits	177		36,935
Other	4,240		5,098
Total other receivables	$	19,235	$	53,661

Inventory (Tables)

3 Months Ended

Mar. 31, 2018

Inventory Disclosure [Abstract]

Schedule of inventories

Inventories as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Raw materials and spare parts	$	35,145	$	37,702
Finished goods	93		90
Total inventories	$	35,238	$	37,792

Land, Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2018

Property, Plant and Equipment [Abstract]

Summary of land, property and equipment

Land, property and equipment as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017			March 31, 2018
Land	$	2,858		$	2,858
LNG liquefaction plants	94,634			94,634
Station equipment	304,090			306,666
Trailers	70,906			71,565
Other equipment	88,313			93,555
Construction in progress	74,905			74,748
	635,706			644,026
Less: accumulated depreciation	(268,401		)	(280,123		)
Total land, property and equipment, net	$	367,305		$	363,903

Accrued Liabilities (Tables)

3 Months Ended

Mar. 31, 2018

Payables and Accruals [Abstract]

Schedule of accrued liabilities

Accrued liabilities as of December 31, 2017 and March 31, 2018 consisted of the following:



	December 31, 2017		March 31, 2018
Accrued alternative fuels incentives (1)	$	2,954	$	14,812
Accrued employee benefits	2,378		2,983
Accrued interest	1,486		35
Accrued gas and equipment purchases	8,722		7,630
Accrued property and other taxes	4,582		5,641
Salaries and wages	8,363		2,787
Other (2)	13,783		11,502
Total accrued liabilities	$	42,268	$	45,390


(1)	Includes the amount of RINs and LCFS Credits and, as of March 31, 2018, the amount of AFTC payable to third parties. The AFTC had expired as of December 31, 2017, but was reinstated in February 2018 for vehicle fuel sales made from January 1, 2017 through December 31, 2017. See Note 17 for more information about AFTC.


(2)	The amount as of December 31, 2017 and March 31, 2018 includes lease termination fees and asset retirement obligations related to the closure of certain fueling stations and working capital adjustments, in the third and fourth quarters of 2017, funding for certain commitments, and transaction fees incurred as a result of the CEC Combination (see Note 4 for more information).

Debt (Tables)

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Schedule of debt and capital lease obligations

Debt and capital lease obligations as of December 31, 2017 and March 31, 2018 consisted of the following and are further discussed below:



	December 31, 2017
	Principal Balances			Unamortized Debt Financing Costs			Balance, Net of Financing Costs
7.5% Notes	$	125,000		$	131		$	124,869
5.25% Notes	110,450			454			109,996
Capital lease obligations	802			—			802
NG Advantage debt	23,437			259			23,178
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



	March 31, 2018
	Principal Balances			Unamortized Debt Financing Costs			Balance Net of Financing Costs
7.5% Notes	$	125,000		$	108		$	124,892
5.25% Notes	110,450			301			110,149
Capital lease obligations	882			—			882
NG Advantage debt	29,437			322			29,115
Other debt	1,188			—			1,188
Total debt and capital lease obligations	266,957			731			266,226
Less amounts due within one year	(141,106		)	(371		)	(140,735		)
Total long-term debt and capital lease obligations	$	125,851		$	360		$	125,491

Net Income Per Share (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Schedule of information required to compute basic and diluted net income (loss) per share

The information required to compute basic and diluted net income per share is as follows:



	Three Months Ended March 31,
	2017	2018
Weighted-average common shares outstanding	148,847,503	152,194,695
Dilutive effect of potential common shares from restricted stock units and stock options	4,124,650	4,448,397
Weighted-average common shares outstanding - diluted	152,972,153	156,643,092

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



	Three Months Ended March 31,
	2017	2018
Stock Options	12,426,603	8,573,749
Convertible Notes	14,991,521	14,991,521
Total	27,418,124	23,565,270

Schedule of at-the-market offering program activity

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



	Three Months Ended March 31,
(in 000s, except per-share amounts)	2017
Gross proceeds	$	10,767
Fees and issuance costs	254
Net proceeds	$	10,513
Shares issued	3,802,500

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

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



	Three Months Ended March 31,
	2017		2018
Stock-based compensation expense, net of $0 tax in 2017 and 2018	$	1,910	$	1,898

General (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in deferred revenue	$ 9,671	$ 3,432
Decrease in accrued expenses	(45,390)	(42,268)
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in restricted cash		1,127
Increase in cash and cash equivalents		$ 1,127
Increase in deferred revenue	$ 175
Decrease in accrued expenses			$ 175

- Definition

The carrying amount of consideration received or receivable as of the balance sheet date on potential earnings that were not recognized as revenue in conformity with GAAP, and which are expected to be recognized as such within one year or the normal operating cycle, if longer, including sales, license fees, and royalties, but excluding interest income.

+ References + Details

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase in opening accumulated deficit	$ 672,641			$ 683,570
Increase in deferred revenue	9,671			3,432
Decrease in notes receivable and other long-term assets, net	(16,590)			(21,397)
Increase in other long-term liabilities	16,381			18,566
Decrease in revenues	(92,251)	$ (76,229)
Decrease in cost of sales	(50,199)	$ (54,597)
Revenue, remaining performance obligation	11,562
Capitalized contract cost, gross	7,123
Capitalized contract cost, accumulated depreciation	3,888
Capitalized contract cost, accumulated amortization	511
Contract with customer, asset, net, receivables	65,687			63,961
Contract with customer, asset, net, current	1,249			1,356
Contract with customer, liability, current	4,110			1,092
Deferred revenue, noncurrent	11,412			$ 13,413
Contract with customer, liability, revenue recognized	1,842
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase in opening accumulated deficit			$ 1,293
Increase in deferred revenue	498
Decrease in notes receivable and other long-term assets, net	963
Increase in other long-term liabilities	498
Decrease in revenues	119
Decrease in cost of sales	$ 119

Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	$ 102,403	$ 89,491
Volume -Related
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	67,219	73,574
Compressor Sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	0	6,467
Station Construction Sales
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	5,798	9,263
Alternative fuels excise tax credit (“AFTC”)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	25,481	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, excluding assessed tax	$ 3,905	$ 187

Divestitures (Details)				3 Months Ended		12 Months Ended
Divestitures (Details)	Jun. 22, 2017 USD ($)	Mar. 31, 2017 USD ($)	Feb. 27, 2017 USD ($) facility	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Acquisition and Divestitures
Gain from sale of certain assets of subsidiary				$ 0	$ 70,648,000
Goodwill				$ 64,328,000		$ 64,328,000	$ 64,328,000
BP Products North America, Inc.
Acquisition 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			$ 123,487,000
Asset purchase agreement, post-closing adjustments, payment	$ 2,010,000
Gain from sale of certain assets of subsidiary						762,000	70,648,000
Asset purchase agreement, contingent consideration (up to)			$ 25,000,000				772,000
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						$ 143,061,000
Goodwill							$ 26,576,000

Investments in Other Entities and Noncontrolling Interest in a Subsidiary (Details) - USD ($)		3 Months Ended
	Jul. 14, 2017	Mar. 31, 2018	Mar. 31, 2017	Feb. 28, 2018	Dec. 31, 2017	Dec. 29, 2017	Oct. 14, 2014	Sep. 16, 2014
Acquisition and Divestitures
Loss from equity method investments		$ 1,468,000	$ 36,000
Loss from noncontrolling interest		1,749,000	335,000
Mansfield
Acquisition and Divestitures
Ownership interest								50.00%
Loss from equity method investments		27,000	36,000
Investment balance		1,485,000			$ 1,512,000
SAFE&CEC S.r.l.
Acquisition and Divestitures
Ownership interest						49.00%
Loss from equity method investments		1,441,000
Investment balance		26,442,000			27,883,000
SAFE&CEC S.r.l. \| Landi Renzo S.p.A.
Acquisition and Divestitures
Ownership interest						51.00%
NG Advantage
Acquisition and Divestitures
Loss from noncontrolling interest		1,749,000	$ 335,000
Noncontrolling interest, fair value		$ 20,919,000			$ 22,668,000
NG Advantage \| Common unit purchase agreement
Acquisition and Divestitures
Ownership interest acquired				53.50%			53.30%
Contingent consideration, liability (up to)				$ 30,000,000
Stock issued during period, acquisitions (in shares)				19,660
NG Advantage \| Common unit purchase agreement \| Preferred units
Acquisition and Divestitures
Stock issued during period, value, acquisitions	$ 7,500,000

Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Cash in excess of insurance limits	$ 43,793,000	$ 34,709,000
Long-term restricted cash	0	0
Held in Escrow
Line of Credit Facility [Line Items]
Short-term restricted cash	750,000
Standby letters of credit
Line of Credit Facility [Line Items]
Short-term restricted cash	$ 1,127,000	$ 1,127,000

Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Short-term investments
Amortized Cost	$ 128,316	$ 141,584
Gross Unrealized Losses	(187)	(122)
Estimated Fair Value	128,129	141,462
Municipal bonds and notes
Short-term investments
Amortized Cost	21,481	21,414
Gross Unrealized Losses	(116)	(49)
Estimated Fair Value	21,365	21,365
Zero coupon bonds
Short-term investments
Amortized Cost	52,153	54,159
Gross Unrealized Losses	(34)	(33)
Estimated Fair Value	52,119	54,126
Corporate bonds
Short-term investments
Amortized Cost	43,737	55,109
Gross Unrealized Losses	(37)	(40)
Estimated Fair Value	43,700	55,069
Certificate of deposits
Short-term investments
Amortized Cost	10,945	10,902
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 10,945	$ 10,902

Fair Value Measurements - Assets and Liabilities (Details) - Fair value measured on recurring basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Warrants
Liabilities:
Warrants	$ 515	$ 536
Municipal bonds and notes
Assets:
Available-for-sale securities	21,365	21,365
Zero coupon bonds
Assets:
Available-for-sale securities	52,119	54,126
Corporate bonds
Assets:
Available-for-sale securities	43,700	55,069
Certificate of deposits
Assets:
Available-for-sale securities	10,945	10,902
Level 1 \| Warrants
Liabilities:
Warrants	0	0
Level 1 \| Municipal bonds and notes
Assets:
Available-for-sale securities	0	0
Level 1 \| Zero coupon bonds
Assets:
Available-for-sale securities	0	0
Level 1 \| Corporate bonds
Assets:
Available-for-sale securities	0	0
Level 1 \| Certificate of deposits
Assets:
Available-for-sale securities	0	0
Level 2 \| Warrants
Liabilities:
Warrants	0	0
Level 2 \| Municipal bonds and notes
Assets:
Available-for-sale securities	21,365	21,365
Level 2 \| Zero coupon bonds
Assets:
Available-for-sale securities	52,119	54,126
Level 2 \| Corporate bonds
Assets:
Available-for-sale securities	43,700	55,069
Level 2 \| Certificate of deposits
Assets:
Available-for-sale securities	10,945	10,902
Level 3 \| Warrants
Liabilities:
Warrants	515	536
Level 3 \| Municipal bonds and notes
Assets:
Available-for-sale securities	0	0
Level 3 \| Zero coupon bonds
Assets:
Available-for-sale securities	0	0
Level 3 \| Corporate bonds
Assets:
Available-for-sale securities	0	0
Level 3 \| Certificate of deposits
Assets:
Available-for-sale securities	$ 0	$ 0

Other Receivables (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Other Receivables
Other receivables	$ 53,661	$ 19,235
Loans to customers to finance vehicle purchases
Other Receivables
Other receivables	5,163	4,746
Accrued customer billings
Other Receivables
Other receivables	6,465	10,072
Fuel tax credits
Other Receivables
Other receivables	36,935	177
Other
Other Receivables
Other receivables	$ 5,098	$ 4,240

Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials and spare parts	$ 37,702	$ 35,145
Finished goods	90	93
Total inventories	$ 37,792	$ 35,238

Land, Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Land, Property and Equipment
Land, property and equipment, gross	$ 644,026		$ 635,706
Less: accumulated depreciation	(280,123)		(268,401)
Total land, property and equipment, net	363,903		367,305
Capitalized software costs	27,379		26,003
Accumulated amortization on the capitalized software costs	19,455		18,737
Amortization expense related to the capitalized software costs	718	$ 872
Property and equipment purchases included in accounts payable and accrued liabilities	939		2,884
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
Station equipment
Land, Property and Equipment
Land, property and equipment, gross	306,666		304,090
Trailers
Land, Property and Equipment
Land, property and equipment, gross	71,565		70,906
Other equipment
Land, Property and Equipment
Land, property and equipment, gross	93,555		88,313
Construction in progress
Land, Property and Equipment
Land, property and equipment, gross	$ 74,748		$ 74,905

Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued alternative fuels incentives	$ 14,812	$ 2,954
Accrued employee benefits	2,983	2,378
Accrued interest	35	1,486
Accrued gas and equipment purchases	7,630	8,722
Accrued property and other taxes	5,641	4,582
Salaries and wages	2,787	8,363
Other	11,502	13,783
Total accrued liabilities	$ 45,390	$ 42,268

Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2013	Jul. 11, 2011
Long-term debt
Principal Balances	$ 266,957	$ 260,931
Unamortized Debt Financing Costs	731	844
Balance, Net of Financing Costs	266,226	260,087
Principal Balances, amounts due within one year	(141,106)	(140,223)
Unamortized Debt Financing Costs, amounts due within one year	(371)	(524)
Balance, Net of Financing Costs, amounts due within one year	(140,735)	(139,699)
Principal Balances, long-term	125,851	120,708
Unamortized Debt Financing Costs, long-term	360	320
Balance, Net of Financing Costs, long-term	125,491	120,388
7.5% Notes
Long-term debt
Principal Balances	125,000	125,000
Unamortized Debt Financing Costs	108	131
Balance, Net of Financing Costs	$ 124,892	124,869
Interest rate	7.50%			7.50%
5.25% Notes
Long-term debt
Principal Balances	$ 110,450	110,450
Unamortized Debt Financing Costs	301	454
Balance, Net of Financing Costs	$ 110,149	109,996
Interest rate	5.25%		5.25%
Capital lease obligations
Long-term debt
Principal Balances	$ 882	802
Unamortized Debt Financing Costs	0	0
Balance, Net of Financing Costs	882	802
NG Advantage debt
Long-term debt
Principal Balances	29,437	23,437
Unamortized Debt Financing Costs	322	259
Balance, Net of Financing Costs	29,115	23,178
Other debt
Long-term debt
Principal Balances	1,188	1,242
Unamortized Debt Financing Costs	0	0
Balance, Net of Financing Costs	$ 1,188	$ 1,242

Debt - 7.5% Notes (Details) - 7.5% Notes				3 Months Ended
Debt - 7.5% Notes (Details) - 7.5% Notes	Feb. 09, 2017 USD ($)	Jun. 14, 2013 USD ($) day $ / shares shares	Jul. 11, 2011 USD ($) note	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Nov. 17, 2017 USD ($)	Feb. 21, 2017 USD ($)	Aug. 27, 2013 USD ($)
Long-term debt
Interest rate			7.50%		7.50%
Financing commitment received (up to)			$ 150,000,000
Number of debt instruments \| note			3
Financing commitment, issuance period			3 years
Principal amount of each debt instrument to be issued			$ 50,000,000
Buyers
Long-term debt
Additional amount of advances under the obligation assumed		$ 50,000,000
Amount of advance funded		$ 50,000,000
Conversion price of shares (in dollars per share) \| $ / shares		$ 15.80
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion		40.00%
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion (at least) \| day		20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion \| day		30
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
Green Energy Investment Holdings, LLC
Long-term debt
Principal amount transferred							$ 11,800,000	$ 5,000,000
Aggregate principal amount					$ 68,200,000
Green Energy Investment Holdings, LLC \| Boone Pickens
Long-term debt
Principal amount transferred						$ 40,000
Debt instrument, repurchase amount during period	$ 25,000,000
Payments for repurchase and retirement of debt instrument	$ 21,750,000
Gain on repurchase of debt instrument				$ 3,191,000
Other Third Parties
Long-term debt
Aggregate principal amount								$ 56,800,000

Debt - 5.25% Notes (Details) - 5.25% Notes	1 Months Ended	8 Months Ended	12 Months Ended
Debt - 5.25% Notes (Details) - 5.25% Notes	Sep. 30, 2013 USD ($) day $ / shares	Dec. 31, 2016 shares	Dec. 31, 2016 USD ($)	Mar. 31, 2018	Mar. 31, 2017 USD ($)
Long-term debt
Interest rate	5.25%			5.25%
Debt issuance amount	$ 250,000,000
Payment of certain debt issuance costs	7,805,000
Amount of advance funded	$ 242,195,000
Conversion rate of debt instrument (shares per USD)	0.0641026
Conversion price of shares (in dollars per share) \| $ / shares	$ 15.60
Number of days within 30 consecutive trading days in which the trade price of the entity's common stock must be at premium of the conversion price for the Company to force conversion (at least) \| day	20
Number of consecutive trading days used to determine the conversion obligation on the notes for the Company to force conversion \| day	30
Percentage of the trade price of common stock that the conversion price must be at premium for the Company to force conversion	160.00%
Redemption price as percentage of principal amount of notes to be redeemed	100.00%
Amount of sinking fund					$ 0
Percentage of principal amount at which notes may be required to be repurchased in event of fundamental change by the entity	100.00%
Payments for repurchase and retirement of debt instrument			$ 84,344,000
Stock issued during period (in shares) \| shares		6,265,829
Debt instrument, repurchase amount during period			$ 139,550,000
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 (at least)	25.00%

Debt - Plains Credit Facility (Details) - Plains Credit Facility - USD ($)	Feb. 29, 2016	Dec. 22, 2016
Long-term debt
Line of credit limit	$ 50,000,000
Period during which the debt instrument principal balance is required to be paid following its issuance	1 year
Long-term Line of Credit		$ 23,500,000
Interest rate	2.70%
LIBOR
Long-term debt
Percentage of margin added to reference rate to determine interest rate on debt	2.30%

- Definition

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

+ References + Details

Debt - Other Debt (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 19, 2014
Trailer And Equipment Debt
Long-term debt
Weighted average interest rate	5.88%	4.98%
Trailer And Equipment Debt \| Maximum
Long-term debt
Interest rate	8.76%
Other Debt
Long-term debt
Weighted average interest rate	4.79%	4.79%
Other Debt \| Maximum
Long-term debt
Interest rate	5.02%
Canton Renewables \| Canton Bonds
Long-term debt
Debt issuance amount			$ 12,400,000

Net Income Per Share (Details) - USD ($)	3 Months Ended
Net Income Per Share (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 21, 2016
Earnings Per Share [Abstract]
Weighted-average common shares outstanding (in shares)	152,194,695	148,847,503
Dilutive effect of potential common shares from restricted stock units and stock options (in shares)	4,448,397	4,124,650
Weighted-average common shares outstanding - diluted (in shares)	156,643,092	152,972,153
Securities Financing Transaction [Line Items]
Fees and issuance costs	$ 0	$ 46,000
Citigroup Global Markets Inc. \| Equity Distribution Agreement
Securities Financing Transaction [Line Items]
Equity financing agreement, amount authorized (up to)			$ 200,000,000
Gross proceeds		10,767,000
Fees and issuance costs		254,000
Net proceeds		$ 10,513,000
Shares issued (in shares)		3,802,500

Net Income Per Share - Anti-dilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Loss Per Share
Anti-dilutive securities (in shares)	23,565,270	27,418,124
Stock Options
Net Loss Per Share
Anti-dilutive securities (in shares)	8,573,749	12,426,603
Convertible Notes
Net Loss Per Share
Anti-dilutive securities (in shares)	14,991,521	14,991,521

Stock-Based Compensation (Details) - USD ($)	3 Months Ended
Stock-Based Compensation (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation expense, net of $0 tax in 2016 and 2017	$ 1,898,000	$ 1,910,000
Stock-based compensation expense, tax	0	$ 0
Unrecognized compensation cost	$ 8,340,000
Unrecognized compensation cost, weighted-average period	2 years 4 months 2 days

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income tax benefit (expense)	$ (88)	$ 2,263
Decrease in deferred tax liability, reduction in goodwill	2,493
Decrease in deferred tax assets, operating loss carryforwards	29,768
Decrease in deferred tax asset valuation allowance	29,768
Unrecognized tax benefits, increase from portion of AFTC revenue offset by the fuel tax	$ 2,689

Alternative Fuels Excise Tax Credit (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 $ / gallon	Dec. 31, 2015 $ / gallon
Alternative Fuels Excise Tax Credit
Federal alternative fuels tax credit - CNG (in dollars per gasoline gallon equivalent)			0.50
Federal alternative fuels tax credit - LNG (in dollars per liquid gallon)			0.50	0.50
Federal alternative fuels tax credit - credits recognized as revenue \| $	$ 25,481	$ 0

Recently Adopted Accounting Changes and Recently Issued Accounting Standards (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in net cash flows	$ 6,743

Subsequent Events (Details) $ / shares in Units, $ in Thousands	May 09, 2018 USD ($) director $ / shares shares	May 08, 2018 shares	Apr. 10, 2018 shares	Mar. 31, 2018 shares	Dec. 31, 2017 shares
Subsequent Event [Line Items]
Common stock, outstanding (in shares)				152,514,550	151,650,969
Subsequent Event \| Purchase Agreement
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares)	50,856,296
Price per share (in dollars per share) \| $ / shares	$ 1.64
Consideration received on transaction \| $	$ 83,404
Common stock, outstanding (in shares)			152,514,550
Percentage of ownership after transaction	25.00%
Number of designated directors \| director	2
Registration statements filing period	60 days
Registration statements effectiveness period	90 days
Liquidation damages, requisite period	30 days
Liquidation damages, percentage of purchase price	0.75%
Subsequent Event \| Purchase Agreement \| Maximum
Subsequent Event [Line Items]
Liquidation damages, percentage of purchase price	4.00%
Subsequent Event \| Purchase Agreement \| Purchase Percentage Covenant, Threshold
Subsequent Event [Line Items]
Percentage of ownership after transaction	5.00%
Subsequent Event \| Purchase Agreement \| Purchase Percentage Covenant, Maximum
Subsequent Event [Line Items]
Percentage of ownership after transaction	30.00%
Subsequent Event \| Purchase Agreement \| Shareholders Elected Fewer Shares
Subsequent Event [Line Items]
Number of shares issued in transaction (in shares)		30,498,520
Consideration received on transaction \| $	$ 50,018
Common stock, outstanding (in shares)		152,568,887
Percentage of ownership after transaction	19.99%	19.99%
Subsequent Event \| Purchase Agreement \| Director Designee Continuation, Term 1
Subsequent Event [Line Items]
Number of designated directors \| director	2
Subsequent Event \| Purchase Agreement \| Director Designee Continuation, Term 1 \| Maximum
Subsequent Event [Line Items]
Percentage of ownership after transaction	16.70%
Subsequent Event \| Purchase Agreement \| Director Designee Continuation, Term 1 \| Minimum
Subsequent Event [Line Items]
Percentage of ownership after transaction	10.00%
Subsequent Event \| Purchase Agreement \| Director Designee Continuation, Term 2
Subsequent Event [Line Items]
Percentage of ownership after transaction	10.00%
Number of designated directors \| director	1