Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (16,743)	$ (24,339)	$ (20,659)
Foreign currency translation adjustment	15	(23)	14
Comprehensive loss	$ (16,728)	$ (24,362)	$ (20,645)