Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Net loss	$ (68,606)	$ (46,447)	$ (42,831)
Change in unrealized gain (loss) on investments	778	59	(84)
Comprehensive loss	$ (67,828)	$ (46,388)	$ (42,915)