Weichai Transactions - Series B Convertible Preferred Stock (Details) - Share Purchase Agreement, Weichai Transaction - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
	Sep. 30, 2017	Nov. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	10.00%
Redeemable Convertible Series B Preferred Stock
Class of Stock [Line Items]
Exercise price of warrants or rights (in dollars per share)					$ 16
Beneficial conversion feature		$ 23.1
Number of shares issued in transaction (in shares)			2,385,624
Issuance of redeemable convertible Series B Preferred Stock, net of fees (in shares)		4,771,248		4,771,000