Weichai Transactions - Weichai Warrant (Details) - USD ($)				1 Months Ended	3 Months Ended		12 Months Ended
	May 03, 2019	Apr. 23, 2019	Oct. 01, 2018	Sep. 30, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Warrant, current					$ 30,700,000			$ 35,100,000
(Gain) loss from change in value of warrants					(4,400,000)	$ (2,500,000)		2,500,000
Purchased inventory					800,000	$ 0
Share Purchase Agreement, Weichai Transaction
Class of Warrant or Right [Line Items]
Exercisable (term)				90 days
Volume weighted average share price percentage			50.00%
Trading period			20 days
Aggregate amount			$ 15,000,000
Expense as a result of the exercise					$ 30,700,000
Warrant, current								$ 35,100,000
Share Purchase Agreement, Weichai Transaction | Subsequent Event
Class of Warrant or Right [Line Items]
Expense as a result of the exercise							$ 1,400,000
Share Purchase Agreement, Second Amended And Restated Warrant | Series B | Subsequent Event
Class of Warrant or Right [Line Items]
Exercise of private placement warrants (in shares)		4,049,759
Percent ownership of outstanding common stock	51.50%
Exercise proceeds for the warrants		$ 1,600,000