Weichai Transactions - Weichai Warrant (Details) - USD ($) $ in Thousands			1 Months Ended	12 Months Ended
	Apr. 23, 2019	Oct. 01, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Exercise proceeds for the warrants				$ 1,616	$ 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
Expense as a result of the exercise	$ 1,400
Share Purchase Agreement, Second Amended And Restated Warrant | Series B Convertible Preferred Stock
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