Net Loss Per Common Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (5,446)	$ (1,444)	$ (16,719)	$ (11,400)
Denominator:
Weighted-average common shares outstanding	12,700,802	3,023,794	12,595,015	3,019,238
Less: weighted-average unvested common shares subjected to repurchase	(140,990)	(366,220)	(169,625)	(429,534)
Weighted-average common shares outstanding used to calculate net loss per common share, basic and diluted	12,559,812	2,657,574	12,425,390	2,589,704
Net loss per common share, basic and diluted	$ (0.43)	$ (0.54)	$ (1.35)	$ (4.40)