Consolidated Statements of Comprehensive Loss (unaudited) - 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	$ (20,923)	$ (11,472)	$ (57,814)	$ (35,878)
Other comprehensive gain (loss):
Net change in unrealized gains (losses) on marketable securities	(7)	(10)	18	(11)
Comprehensive loss	$ (20,930)	$ (11,482)	$ (57,796)	$ (35,889)