Net Loss Per Share Available to Common Stockholders - Potential Common Shares Outstanding (Detail) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding	5,204	21,946
Convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding		17,275
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding	4,990	4,274
Common stock subject to repurchase [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding	214	309
Convertible preferred stock warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding		51
Common stock warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares outstanding		37