Selected Quarterly Data - Summarized Quarterly Information (Detail) - 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	$ 9,890	$ 12,335	$ 15,078	$ 14,817	$ 14,018	$ 12,810	$ 11,638	$ 11,876	$ 52,120	$ 50,342	$ 31,402
Net loss	(9,859)	(12,300)	(15,106)	(14,758)	(13,954)	(12,798)	(10,167)	(10,748)	(52,023)	(47,667)	(32,718)
Net loss attributable to common stockholders	$ (9,859)	$ (12,300)	$ (15,106)	$ (14,758)	$ (13,954)	$ (12,798)	$ (16,251)	$ (13,818)	$ (52,023)	$ (56,821)	$ (39,085)
Basic and diluted net loss per share attributable to common stockholders (usd per share)	$ (0.34)	$ (0.51)	$ (0.63)	$ (0.62)	$ (0.59)	$ (0.59)	$ (0.91)	$ (24.49)	$ (2.07)	$ (3.54)	$ (74.86)