CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Allowance for doubtful accounts	$ 134	$ 141
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock shares issued on exercise of options (in shares)	31,120,608	29,943,086
Common stock, shares outstanding (in shares)	31,120,608	29,943,086
Series C Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock, par value (in dollars per share)	$ 0.10	$ 0.10
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	6,878	9,968
Preferred stock, shares outstanding (in shares)	6,878	9,968