Net Loss Per Share - Schedule of Antidilutive Securities Excluded from Net Loss per Share Computation (Details) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Option to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	22,707	18,161
Unvested early exercised common shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	5,240	8,554
Unvested restricted stock awards and units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	2,140	0
Conversion of preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	0	69,473
Warrants to purchase convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	0	616
Warrants to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from net loss per share (shares)	616	0