CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Treasury Stock
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)
Beginning balance at Dec. 31, 2019	28,497	23	165,527	(126,912)	(10,141)
Increase (Decrease) in Stockholders' Equity
Net loss	(18,454)			(18,454)
Stock-based compensation expense	317		330		(13)
Payment of withholding taxes for net settlement of stock-based awards	(5)		(5)
Ending balance at Jun. 30, 2020	10,355	23	165,852	(145,366)	(10,154)
Beginning balance at Mar. 31, 2020	27,941	23	165,691	(127,624)	(10,149)
Increase (Decrease) in Stockholders' Equity
Net loss	(17,742)			(17,742)
Stock-based compensation expense	160		165		(5)
Payment of withholding taxes for net settlement of stock-based awards	(4)		(4)
Ending balance at Jun. 30, 2020	$ 10,355	$ 23	$ 165,852	$ (145,366)	$ (10,154)