Selected Quarterly Financial Data - Quarterly Financial 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]
Total revenue	$ 565	$ 67	$ 114	$ 244	$ 375	$ 539	$ 761	$ 568	$ 990	$ 2,243	$ 1,334
Total operating expenses	10,403	9,426	8,516	7,841	7,470	11,141	8,806	7,757	36,186	35,174	17,812
Loss from operations	(9,838)	(9,359)	(8,402)	(7,597)	(7,095)	(10,602)	(8,045)	(7,189)	(35,196)	(32,931)	(16,478)
Net loss	(10,329)	(9,693)	(6,906)	(6,524)	(8,139)	(10,674)	(8,121)	(7,222)	(33,452)	(34,156)	(18,025)
Net loss applicable to common stockholders	$ (10,329)	$ (9,693)	$ (6,906)	$ (6,524)	$ (8,139)	$ (10,674)	$ (8,121)	$ (7,741)	$ (33,452)	$ (34,675)	$ (21,882)
Net loss per share applicable to common stockholders-basic and diluted (in dollars per share)	$ (0.53)	$ (0.50)	$ (0.36)	$ (0.36)	$ (0.49)	$ (0.66)	$ (0.51)	$ (0.80)	$ (1.76)	$ (2.37)	$ (16.18)