Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,482,470	5,184,633	5,026,246
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,125,692	5,069,133	5,026,246
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	356,778	115,500