Summary of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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	$ 0	$ 0	$ 0	$ 19,769	$ 0	$ 0	$ 0	$ 0	$ 19,769	$ 0	$ 2,919
Operating expenses	30,748	35,844	25,322	22,057	20,468	16,857	14,361	12,725	113,971	64,411	52,597
Net loss	$ (29,315)	$ (34,435)	$ (23,987)	$ (1,192)	$ (19,444)	$ (15,875)	$ (13,381)	$ (11,842)	$ (88,929)	$ (60,542)	$ (46,090)
Net loss per share:
Basic and Diluted	$ (0.34)	$ (0.40)	$ (0.28)	$ (0.01)	$ (0.26)	$ (0.21)	$ (0.18)	$ (0.17)	$ (1.05)	$ (0.81)
Shares used in the calculation of net loss per share:
Basic and Diluted	86,184	85,856	85,518	80,349	76,139	75,948	75,879	70,260	84,496	74,577