Basic and Diluted Net Loss Per Share - Summary of Calculation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator
Net loss	$ (34,918)	$ (39,677)	$ (106,754)	$ (117,275)
Denominator
Weighted-average number of common shares used in net loss per share calculation	80,297,757	78,866,597	80,017,436	74,716,970
Less: Weighted-average shares subject to repurchase	0	0	0	(686)
Denominator: basic and diluted	80,297,757	78,866,597	80,017,436	74,716,284
Net loss per share, basic and diluted (in dollars per share)	$ (0.43)	$ (0.50)	$ (1.33)	$ (1.57)