Weichai Transactions - Weichai Warrant (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Apr. 23, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Class of Warrant or Right [Line Items]
Exercise proceeds for the warrants			$ 0	$ 1,616
Share Purchase Agreement, Weichai Transaction
Class of Warrant or Right [Line Items]
Exercisable (in years)		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