Weichai Transactions - Weichai Warrant (Details) - USD ($) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended
	May 03, 2019	Apr. 23, 2019	Oct. 01, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Exercise proceeds for the warrants							$ 1,616	$ 0
Warrant liability					$ 0		0		$ 35,100
Loss from change in value and exercise of warrants					0	$ 10,200	1,352	19,200
Inventory purchased					$ 600	$ 1,000	$ 1,500	$ 1,200
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
Warrant liability									$ 35,100
Share Purchase Agreement, Second Amended And Restated Warrant | Series B
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