Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Were Excluded From The Calculation of Diluted Net Loss Per Share Due to Their Anti-Dilutive Effect (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Outstanding Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive securities excluded from the calculation of diluted net loss per share due to anti-dilutive effect (in shares)	7,019,083	5,574,179	4,443,317
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive securities excluded from the calculation of diluted net loss per share due to anti-dilutive effect (in shares)	253,100	214,300	508,800