Net Loss Per Share Attributable to Common Stockholders - Computation of 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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss attributable to common stockholders	$ (4,570)	$ (4,112)	$ (17,966)	$ (18,163)
Denominator:
Weighted-average common shares outstanding - basic and diluted	44,785	40,120	43,830	39,240
Net loss per common share - basic and diluted	$ (0.10)	$ (0.10)	$ (0.41)	$ (0.46)