Basic and Diluted Net Loss Per Share - Outstanding Shares of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	32,690,995	24,202,653
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	10,747,216	11,142,893
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	0	584
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	1,902,783	2,218,578
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	1,250,000	1,250,000
Shares of common stock to be issued upon conversion of the Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares of potentially dilutive securities	18,790,996	9,590,598