Consolidated Statements of Comprehensive Loss (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (11,472)	$ (11,471)	$ (35,878)	$ (33,340)
Other comprehensive gain (loss):
Net change in unrealized gains (losses) on marketable securities	(10)	(1)	(11)	11
Comprehensive loss	$ (11,482)	$ (11,472)	$ (35,889)	$ (33,329)