Net Loss Per Common Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (5,308)	$ (1,444)	$ (5,416)	$ (4,540)	$ (6,253)	$ (4,509)	$ (3,268)	$ (4,153)	$ (16,708)	$ (18,183)	$ (10,183)
Denominator:
Weighted-average common shares outstanding									5,117,226	3,013,743	2,635,621
Less: weighted-average unvested common shares subjected to repurchase									(378,730)	(461,538)	(91,043)
Weighted-average common shares outstanding used to calculate net loss per common share, basic and diluted									4,738,496	2,552,205	2,544,578
Net loss per common share, basic and diluted	$ (0.48)	$ (0.54)	$ (2.12)	$ (1.78)	$ (2.45)	$ (1.77)	$ (1.28)	$ (1.63)	$ (3.53)	$ (7.12)	$ (4.00)