Net Loss per Share - Anti-Dilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	4,067,998	3,245,075
Common stock subject to repurchase
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	27,073	246,079
Outstanding options
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	4,040,925	2,998,996