Basic and Diluted Net Loss Per Share - Summary of Calculation of Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator
Net loss	$ (39,677)	$ (30,703)	$ (117,275)	$ (83,053)
Denominator
Weighted-average number of common shares used in net loss per share calculation	78,866,597	64,823,095	74,716,970	62,802,684
Less: Weighted-average shares subject to repurchase	0	(11,463)	(686)	(20,016)
Denominator: basic and diluted	78,866,597	64,811,632	74,716,284	62,782,668
Net loss per share, basic and diluted	$ (0.50)	$ (0.47)	$ (1.57)	$ (1.32)