Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (59,432)	$ (37,232)	$ (25,040)
Other comprehensive loss:
Unrealized gain (loss) on investments	20	(22)
Comprehensive loss	$ (59,412)	$ (37,254)	$ (25,040)