Net Loss per Share - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss - basic and diluted	$ (11,643)	$ (14,301)
Denominator:
Weighted average common shares outstanding	29,326,075	22,867,843
Less: weighted average common shares subject to repurchase	(33,940)	(287,223)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	29,292,135	22,580,620
Net loss per share, basic and diluted (in dollars per share)	$ (0.40)	$ (0.63)