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	$ (25,621)	$ (31,553)	$ (41,007)
Other comprehensive loss
Unrealized loss on available-for-sale securities	(16)	(94)	(2)
Total comprehensive loss	$ (25,637)	$ (31,647)	$ (41,009)