CONDENSED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (15,180)	$ (5,597)	$ (39,478)	$ (19,717)
Net unrealized (loss) gain on marketable securities	(42)	(35)	422	95
Comprehensive loss	$ (15,222)	$ (5,632)	$ (39,056)	$ (19,622)