CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Treasury Stock
Beginning balance at Dec. 31, 2018	$ (18,578)	$ 19	$ 119,724	$ (135,160)	$ (3,161)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	8,248			8,248
Stock-based compensation expense	1,248		1,248
Common stock issued for stock-based awards, net	(489)		(2,309)		1,820
Issuance of common stock to Weichai	38,068	4	38,064
Ending balance at Dec. 31, 2019	28,497	23	156,727	(126,912)	(1,341)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(22,982)			(22,982)
Stock-based compensation expense	607		535		72
Common stock issued for stock-based awards, net	(25)				(25)
Ending balance at Dec. 31, 2020	$ 6,097	$ 23	$ 157,262	$ (149,894)	$ (1,294)