Weichai Transactions - Valuation and Accounting for Issuance and Conversion of Common Stock, Preferred Stock (and Related Beneficial Conversion Feature) and Warrants (Details) - USD ($)			3 Months Ended								12 Months Ended
	Oct. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Subsidiary, Sale of Stock [Line Items]
Loss (gain) from change in fair value of warrants			$ 1,500,000	$ 1,700,000	$ 800,000	$ 0	$ 0	$ 0	$ (157,000)	$ (1,256,000)	$ 4,000,000	$ (1,413,000)	$ (9,300,000)	$ (6,170,000)
Carrying value of the common stock and series b preferred stock			39,300,000								39,300,000
Aggregate remaining proceeds											38,700,000
Proceeds allocated based on related fair value, net of transaction costs											24,600,000
Transaction costs											(400,000)
Deemed dividend recorded in additional paid in capital											37,800,000
Carrying value of preferred stock		$ 24,617,000	62,900,000	$ 31,411,000	$ 27,807,000	24,617,000					62,900,000
Par value (in dollars per share)			0.001								0.001
Conversion price (in dollars per share)	$ 4.85
Share Purchase Agreement, Weichai Transaction
Subsidiary, Sale of Stock [Line Items]
Weichai warrant liability		$ 20,700,000				$ 20,700,000
Warrant, current			$ 24,700,000								24,700,000
Redeemable Convertible Series B Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Proceeds allocated to Series B Preferred Stock											25,000,000
Fully accreted discount											$ 14,100,000
Redeemable Convertible Series B Preferred Stock | Share Purchase Agreement, Weichai Transaction
Subsidiary, Sale of Stock [Line Items]
Beneficial conversion feature	$ 23,100,000
Issuance of redeemable convertible Series B Preferred Stock, net of fees (in shares)		4,771,248									4,771,000
Common stock
Subsidiary, Sale of Stock [Line Items]
Proceeds allocated to common stock											$ 14,300,000