Net Loss Per Share Applicable to Common Stockholders - Common Stock Excluded from Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	23,452,549	23,219,492	23,452,549	23,219,492
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	19,731,564	19,731,564	19,731,564	19,731,564
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	3,657,145	3,287,171	3,657,145	3,287,171
Unvested Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	63,840	200,757	63,840	200,757