Condensed Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (26,875)	$ (17,595)	$ (50,097)	$ (31,701)
Other comprehensive loss, net of tax:
Unrealized gain on investments	2		30	6
Comprehensive loss	$ (26,873)	$ (17,595)	$ (50,067)	$ (31,695)