Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (1,828)	$ (6,775)	$ (10,519)	$ (12,444)
Change in unrealized gain (loss) on available-for-sale- securities, net of tax	12	(24)	24	5
Other comprehensive income (loss)	12	(24)	24	5
Comprehensive loss, net of income tax	$ (1,816)	$ (6,799)	$ (10,495)	$ (12,439)