Significant Accounting Policies - Schedule of Diluted Net Loss Per Share Attributable to Common Stockholders Due to Anti-Dilutive Effect (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share	14,818,518	26,669,830
Series A convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share		16,930,668
Series A-1 convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share		2,250,000
Unvested restricted common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share	7,598,095	7,489,162
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share	3,840,931
Estimated shares issued if success payment valuation occurred [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the calculation of diluted net loss per share	3,379,492