CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Treasury Stock
Beginning balance at Dec. 31, 2017	$ 32,172	$ 19	$ 123,838	$ (82,147)	$ (9,538)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(44,828)			(44,828)
Stock-based compensation expense	1,963		2,389		(426)
Payment of withholding taxes for net settlement of stock-based awards	(289)		(289)
Ending balance at Sep. 30, 2018	(9,269)	19	125,938	(125,262)	(9,964)
Beginning balance at Jun. 30, 2018	8,925	19	125,272	(106,443)	(9,923)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(18,819)			(18,819)
Stock-based compensation expense	545		586		(41)
Payment of withholding taxes for net settlement of stock-based awards	80		80
Ending balance at Sep. 30, 2018	(9,269)	19	125,938	(125,262)	(9,964)
Beginning balance at Dec. 31, 2018	(18,578)	19	126,412	(135,160)	(9,849)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	172			172
Stock-based compensation expense	1,101		1,390		(289)
Payment of withholding taxes for net settlement of stock-based awards	(486)		(486)
Exercise of Weichai Warrant	38,068	4	38,064
Ending balance at Sep. 30, 2019	20,277	23	165,380	(134,988)	(10,138)
Beginning balance at Jun. 30, 2019	14,391	23	165,437	(140,744)	(10,325)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	5,756			5,756
Stock-based compensation expense	173		(14)		187
Payment of withholding taxes for net settlement of stock-based awards	(43)		(43)
Ending balance at Sep. 30, 2019	$ 20,277	$ 23	$ 165,380	$ (134,988)	$ (10,138)