Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (8,755)	$ (9,865)	$ (23,636)	$ (28,271)
Other comprehensive loss
Unrealized (loss) gain on available-for-sale securities	(3)	4	(3)	2
Total comprehensive loss	$ (8,758)	$ (9,861)	$ (23,639)	$ (28,269)