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