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 loss	(26,009)			(26,009)
Stock-based compensation expense	1,418		1,803		(385)
Payment of withholding taxes for net settlement of stock-based awards	(369)		(369)
Ending balance at Jun. 30, 2018	8,925	19	125,272	(106,443)	(9,923)
Beginning balance at Mar. 31, 2018	20,480	19	124,981	(94,638)	(9,882)
Increase (Decrease) in Stockholders' Equity
Net loss	(11,805)			(11,805)
Stock-based compensation expense	619		660		(41)
Payment of withholding taxes for net settlement of stock-based awards	(369)		(369)
Ending balance at Jun. 30, 2018	8,925	19	125,272	(106,443)	(9,923)
Beginning balance at Dec. 31, 2018	(18,578)	19	126,412	(135,160)	(9,849)
Increase (Decrease) in Stockholders' Equity
Net loss	(5,584)			(5,584)
Stock-based compensation expense	928		1,404		(476)
Payment of withholding taxes for net settlement of stock-based awards	(443)		(443)
Exercise of Weichai Warrant	38,068	4	38,064
Ending balance at Jun. 30, 2019	14,391	23	165,437	(140,744)	(10,325)
Beginning balance at Mar. 31, 2019	(20,975)	19	126,977	(137,746)	(10,225)
Increase (Decrease) in Stockholders' Equity
Net loss	(2,998)			(2,998)
Stock-based compensation expense	393		493		(100)
Payment of withholding taxes for net settlement of stock-based awards	(97)		(97)
Exercise of Weichai Warrant	38,068	4	38,064
Ending balance at Jun. 30, 2019	$ 14,391	$ 23	$ 165,437	$ (140,744)	$ (10,325)