Outstanding Shares of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Detail)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	28,257,450	10,656,336
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	10,637,489	9,418,532
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	24,151	75,136
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	955,310	162,668
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	1,500,000	1,000,000
Shares of common stock to be issued upon conversion of the Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total outstanding shares of potentially dilutive securities	15,140,500