Weichai Transactions - Weichai Warrant (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	May 03, 2019	Apr. 23, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2016
Share Purchase Agreement, Weichai Transaction
Class of Warrant or Right [Line Items]
Exercisable (term)			90 days
Volume weighted average share price percentage			85.00%	50.00%
Trading period			20 days
EBITDA, minimum (less than)					$ 22.0
Net book value per share, minimum (less than) (in dollars per share)					$ 8
Aggregate amount					$ 15.0
Subsequent Event | Share Purchase Agreement, Weichai Transaction
Class of Warrant or Right [Line Items]
Expense as a result of the exercise		$ 1.4
Subsequent Event | 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		$ 1.6