Weichai Transactions (Details) - USD ($) $ in Thousands				1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 23, 2019	Oct. 01, 2018	Mar. 20, 2017	Sep. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Class of Warrant or Right [Line Items]
Gain from change in fair value of warrants					$ 0	$ 4,400
Term of collaborative arrangement			3 years
Purchased inventory					5,100	$ 800
Outstanding payables					$ 11,100		$ 5,900
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	20 days
Aggregate amount (up to)		$ 15,000
Expense as a result of the exercise							$ 1,400
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