Stockholders' Equity - Common and Treasury Stock and Series B Convertible Preferred Stock (Details)		1 Months Ended	12 Months Ended
	Apr. 23, 2019 shares	Mar. 31, 2017 shares	Dec. 31, 2019 series_of_preferred $ / shares shares	Dec. 31, 2018 $ / shares shares
Common Shares Issued
Common stock, issued, beginning balance (in shares)			19,067,000	19,067,000
Common stock, issued, ending balance (in shares)			23,117,000	19,067,000
Treasury Stock Shares
Treasury stock shares, beginning balance (in shares)			429,000	634,000
Treasury stock shares issued (in shares)			(169,000)	(205,000)
Treasury stock shares, ending balance (in shares)			260,000	429,000
Common Shares Outstanding
Common stock, outstanding, beginning balance (in shares)			18,638,000	18,433,000
Common stock, outstanding, ending balance (in shares)			22,857,000	18,638,000
Series B convertible preferred stock, authorized (in shares)			5,000,000	5,000,000
Series B convertible preferred stock, par value (in dollars per share) | $ / shares			$ 0.001	$ 0.001
Preferred stock, outstanding (in shares)			0	0
Common Stock
Common Shares Outstanding
Conversion of outstanding stock (in shares)			169,000	205,000
Series B Convertible Preferred Stock
Common Shares Outstanding
Series B convertible preferred stock, authorized (in shares)			5,000,000
Series B convertible preferred stock, par value (in dollars per share) | $ / shares			$ 0.001
Number of authorized series of preferred stock | series_of_preferred			2
Share Purchase Agreement, Weichai Transaction | Common Stock
Common Shares Issued
Shares issued to Weichai (in shares)		2,728,752	4,050,000
Common Shares Outstanding
Shares issued to Weichai (in shares)		2,728,752	4,050,000
Share Purchase Agreement, Second Amended And Restated Warrant | Series B Convertible Preferred Stock
Common Shares Outstanding
Warrants exercised (in shares)	4,049,759
Percent ownership of outstanding common stock	51.50%