Fair Value of Financial Instruments - Schedule of Financial Instruments Measured at Fair Value (Details) - Weichai Warrant liability - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weichai Warrant liability	$ 0	$ 0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weichai Warrant liability	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weichai Warrant liability	$ 35,100	$ 24,700