Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 20,050	$ 19,678	$ 10,889	$ 10,896	$ 10,607	$ 9,584	$ 7,840	$ 9,909	$ 61,513	$ 37,940	$ 15,933
Loss from operations	(20,050)	(19,678)	(10,889)	(10,896)	(10,607)	(9,584)	(7,840)	(9,909)	(61,513)	(37,940)	(15,933)
Other income (expense), net	193	203	200	201	237	236	222	212
Net loss	$ (19,857)	$ (19,475)	$ (10,689)	$ (10,695)	$ (10,370)	$ (9,348)	$ (7,618)	$ (9,697)	$ (60,716)	$ (37,034)	$ (13,167)
Net loss per share applicable to common stockholders—basic and diluted	$ (0.66)	$ (0.68)	$ (0.40)	$ (0.53)	$ (0.52)	$ (0.47)	$ (0.39)	$ (43.37)	$ (2.29)	$ (8.04)	$ (126.94)