Selected Quarterly Financial Data (Unaudited) - CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity
Beginning balance	$ (9,269)	$ 8,925	$ 20,480	$ 32,172	$ 6,129	$ 18,210	$ 30,048	$ 40,660	$ 32,172	$ 40,660	$ 32,172	$ 40,660	$ 32,172	$ 40,660
Net loss	(9,898)	(18,819)	(11,805)	(14,204)	(6,129)	(9,201)	(7,747)	(24,535)	(26,009)	(32,282)	(44,828)	(41,483)	(54,726)	(47,612)
Stock-based compensation expense	700	545	619	799	771	731	(162)	(155)					2,663	1,183
Payment of withholding taxes for net settlement of stock-based awards	(111)	80	(369)		(9)	(8)	(27)
Issuance of common stock to Weichai, net of fees								14,078						14,078
Issuance of Series B convertible preferred stock, net of fees								0						0
Settlement of equity-related acquisition consideration							(712)							(712)
Deemed dividend on Series B convertible preferred stock					(31,067)	(3,603)	(3,190)							(37,860)
Conversion of Series B convertible preferred stock to common stock					62,477									62,477
Ending balance	(18,578)	(9,269)	8,925	20,480	32,172	6,129	18,210	30,048	8,925	18,210	(9,269)	6,129	(18,578)	32,172
ASC 606
Increase (Decrease) in Stockholders' Equity
Cumulative effect of adoption of ASC 606															$ 1,713
Series B Convertible Preferred Stock
Increase (Decrease) in Stockholders' Equity
Beginning balance				0	31,410	27,807	24,617	0	0	0	0	0	0	0
Issuance of Series B convertible preferred stock, net of fees								24,617						24,617
Deemed dividend on Series B convertible preferred stock					31,067	3,603	3,190							37,860
Conversion of Series B convertible preferred stock to common stock					(62,477)									(62,477)
Ending balance	0				0	31,410	27,807	24,617		27,807		31,410	0	0
Common Stock
Increase (Decrease) in Stockholders' Equity
Beginning balance	19	19	19	19	14	14	14	12	19	12	19	12	19	12
Issuance of common stock to Weichai, net of fees								2						2
Conversion of Series B convertible preferred stock to common stock					5									5
Ending balance	19	19	19	19	19	14	14	14	19	14	19	14	19	19
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Beginning balance	125,938	125,272	124,981	123,838	91,737	96,543	100,686	86,764	123,838	86,764	123,838	86,764	123,838	86,764
Stock-based compensation expense	585	586	660	1,143	705	(1,195)	(214)	(154)					2,974	(860)
Payment of withholding taxes for net settlement of stock-based awards	(111)	80	(369)		(9)	(8)	(27)
Issuance of common stock to Weichai, net of fees								14,076						14,076
Settlement of equity-related acquisition consideration							(712)							(712)
Deemed dividend on Series B convertible preferred stock					(31,067)	(3,603)	(3,190)							(37,860)
Conversion of Series B convertible preferred stock to common stock					62,472									62,472
Ending balance	126,412	125,938	125,272	124,981	123,838	91,737	96,543	100,686	125,272	96,543	125,938	91,737	126,412	123,838
Treasury Stock
Increase (Decrease) in Stockholders' Equity
Beginning balance	(9,964)	(9,923)	(9,882)	(9,538)	(9,604)	(11,530)	(11,582)	(11,581)	(9,538)	(11,581)	(9,538)	(11,581)	(9,538)	(11,581)
Stock-based compensation expense	115	(41)	(41)	(344)	66	1,926	52	(1)					(311)	2,043
Ending balance	(9,849)	(9,964)	(9,923)	(9,882)	(9,538)	(9,604)	(11,530)	(11,582)	(9,923)	(11,530)	(9,964)	(9,604)	(9,849)	(9,538)
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Beginning balance	(125,262)	(106,443)	(94,638)	(82,147)	(76,018)	(66,817)	(59,070)	(34,535)	(82,147)	(34,535)	(82,147)	(34,535)	(82,147)	(34,535)
Net loss	(9,898)	(18,819)	(11,805)	(14,204)	(6,129)	(9,201)	(7,747)	(24,535)					(54,726)	(47,612)
Ending balance	$ (135,160)	$ (125,262)	$ (106,443)	$ (94,638)	$ (82,147)	$ (76,018)	$ (66,817)	$ (59,070)	$ (106,443)	$ (66,817)	$ (125,262)	$ (76,018)	$ (135,160)	$ (82,147)
Accumulated Deficit | ASC 606
Increase (Decrease) in Stockholders' Equity
Cumulative effect of adoption of ASC 606															$ 1,713