Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Summary of Calculation of Basic and Diluted Net Loss Per Share (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
Numerator
Net loss	$ (34,693)	$ (34,918)	$ (37,171)	$ (34,665)	$ (44,866)	$ (39,677)	$ (42,917)	$ (34,681)	$ (141,447)	$ (162,141)	$ (116,389)
Denominator
Weighted Average Number of Shares Issued, Basic									80,165,402	75,879,721	64,228,387
Less: Weighted-average shares subject to repurchase									0	(513)	(16,429)
Denominator: basic and diluted	80,600,000	80,298,000	80,098,000	79,650,000	79,330,000	78,867,000	75,963,000	69,213,000	80,165,402	75,879,208	64,211,958
Net loss per share, basic and diluted (in dollars per share)	$ (0.43)	$ (0.43)	$ (0.46)	$ (0.44)	$ (0.57)	$ (0.50)	$ (0.56)	$ (0.50)	$ (1.76)	$ (2.14)	$ (1.81)