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