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, 2017	$ 32,172	$ 19	$ 123,838	$ (82,147)	$ (9,538)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(54,726)			(54,726)
Stock-based compensation expense	2,663		2,974		(311)
Payment of withholding taxes for net settlement of stock-based awards	(400)		(400)
Ending balance at Dec. 31, 2018	(18,578)	19	126,412	(135,160)	(9,849)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	8,248			8,248
Stock-based compensation expense	1,248		1,540		(292)
Payment of withholding taxes for net settlement of stock-based awards	(489)		(489)
Issuance of common stock to Weichai	38,068	4	38,064
Ending balance at Dec. 31, 2019	$ 28,497	$ 23	$ 165,527	$ (126,912)	$ (10,141)