Net Loss Per Common Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net loss	$ (7,369)	$ (5,187)	$ (8,646)	$ (11,273)
Denominator:
Weighted-average common shares outstanding	16,689,677	12,583,325	15,786,043	12,541,245
Less: weighted-average unvested common shares subjected to repurchase	(54,416)	(169,801)	(68,795)	(184,179)
Weighted-average common shares outstanding used to calculate net loss per common share, basic and diluted	16,635,261	12,413,524	15,717,248	12,357,066
Net loss per common share, basic and diluted	$ (0.44)	$ (0.42)	$ (0.55)	$ (0.91)