Summary of Significant Accounting Policies - Potentially Dilutive Securities Excluded from Calculation of Diluted Net (Loss) Income per Share Due to Anti-Dilutive Effect (Detail) - shares	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	9,853,841		11,348,796		6,470,038
Options to Purchase Common Stock, Including Purchases from Contributions to ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	9,304,018		10,731,164		6,320,038
Unvested Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	368,927		440,354		0
Restricted Stock Awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	30,896		27,278		0
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	150,000	[1]	150,000	[1]	150,000	[1]	150,000

[1] In 2012, the Company issued a warrant with a ten-year term to purchase 150,000 shares of its common stock at an exercise price of $20.00 per share.