CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,418)	$ (14,376)	$ (49,099)	$ (49,405)
Other comprehensive (losses) gains:
Unrealized (losses) gains on available-for-sale investments, net of taxes of zero and $137 for the three months ended September 30, 2017 and 2016, respectively, and zero and $(2,126) for the nine months ended September 30, 2017 and 2016, respectively	(9)	260	(131)	(4,051)
Comprehensive loss	$ (13,427)	$ (14,116)	$ (49,230)	$ (53,456)