Selected Quarterly Financial Data (Unaudited) (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]
Revenues	$ 10,860	$ 1,865	$ 3,834	$ 4,200	$ 4,219	$ 1,083	$ 736	$ 1,631	$ 20,759	$ 7,669	$ 19,569
Operating expenses	18,221	15,210	25,015	17,071	13,752	12,742	13,749	12,336	75,517	52,579	37,371
Net loss	$ (7,226)	$ (13,000)	$ (21,035)	$ (14,487)	$ (22,168)	$ (9,798)	$ (11,973)	$ (12,741)	$ (55,748)	$ (56,680)	$ (18,668)
Basic and diluted net loss per share (dollars per share)	$ (0.14)	$ (0.25)	$ (0.41)	$ (0.29)					$ (1.08)	$ (1.29)	$ (0.49)
Basic net loss per share (dollars per share)					$ (0.47)	$ (0.23)	$ (0.28)	$ (0.30)
Diluted net loss per share (dollars per share)					$ (0.47)	$ (0.26)	$ (0.29)	$ (0.30)