CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (14,192)	$ (13,418)	$ (41,359)	$ (49,099)
Other comprehensive income (loss)
Unrealized gains (losses) on available-for-sale investments, net of taxes of zero for the three and nine months ended September 30, 2018 and 2017	91	(9)	(229)	(131)
Comprehensive loss	$ (14,101)	$ (13,427)	$ (41,588)	$ (49,230)