Net Loss Per Share - Computation of Potentially Dilutive Securities Outstanding Excluded from Computation of Diluted Shares (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	7,119,929	6,860,878	6,746,848
Unreleased Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	1,010,123	720,840	345,555
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	2,529,740	2,559,972	2,821,227
Convertible Senior Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	1,790,033	1,790,033	1,790,033
Warrants Related to the Issuance of Convertible Senior Notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	1,790,033	1,790,033	1,790,033