Net Loss per Share - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss - basic and diluted	$ (11,047)	$ (11,004)	$ (12,978)	$ (11,643)	$ (10,509)	$ (10,498)	$ (11,631)	$ (14,301)	$ (46,672)	$ (46,939)	$ (55,663)
Denominator:
Weighted average common shares outstanding									29,364,535	27,686,909	20,958,557
Less: weighted average common shares subject to repurchase									(14,725)	(176,949)	(470,051)
Weighted average common shares outstanding used to compute basic and diluted net loss per share									29,349,810	27,509,960	20,488,506
Net loss per share, basic and diluted (in dollars per share)	$ (0.38)	$ (0.37)	$ (0.44)	$ (0.40)	$ (0.36)	$ (0.36)	$ (0.40)	$ (0.63)	$ (1.59)	$ (1.71)	$ (2.72)