Summary of Calculation of Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator
Net loss	$ (33,336)	$ (30,703)	$ (25,825)	$ (26,525)	$ (24,610)	$ (22,503)	$ (19,239)	$ (16,780)	$ (116,389)	$ (83,132)	$ (53,901)
Denominator
Weighted-average number of common shares used in net loss per share calculation									64,228,387	60,570,891	40,132,125
Less: Weighted-average shares subject to repurchase									(16,429)	(61,843)	(198,112)
Denominator: basic and diluted	68,453,000	64,812,000	61,958,000	61,543,000	60,873,000	60,678,000	60,378,000	60,101,000	64,211,958	60,509,048	39,934,013
Net loss per share, basic and diluted	$ (0.49)	$ (0.47)	$ (0.42)	$ (0.43)	$ (0.40)	$ (0.37)	$ (0.32)	$ (0.28)	$ (1.81)	$ (1.37)	$ (1.35)