Condensed 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	$ (11,917)	$ (7,874)	$ (29,450)	$ (20,753)
Other comprehensive income (loss):
Unrealized gain (loss) on marketable securities	23	(3)	69	(28)
Total comprehensive loss	$ (11,894)	$ (7,877)	$ (29,381)	$ (20,781)