Condensed Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,553)	$ (23,221)	$ (24,977)	$ (50,299)
Other comprehensive income (loss):
Unrealized gain (loss) on available for sale securities	50	(9)	54	(78)
Comprehensive loss	$ (13,503)	$ (23,230)	$ (24,923)	$ (50,377)