Net Loss Per Share - Reconciliation of the Numerator and Denominator used in Computing Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (4,081)	$ (4,158)	$ (7,068)	$ (7,070)	$ (7,732)	$ (14,442)	$ (15,307)	$ (29,244)
Denominator:
Weighted-average shares outstanding — basic and diluted	21,961			21,403			21,738	21,287
Net loss per share — basic and diluted	$ (0.19)			$ (0.33)			$ (0.70)	$ (1.37)