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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net loss attributable to common stockholders	$ (5,297)	$ (6,912)	$ (13,396)	$ (14,051)
Denominator:
Weighted-average common shares outstanding - basic and diluted	43,921	39,175	43,345	38,793
Net loss per common share - basic and diluted	$ (0.12)	$ (0.18)	$ (0.31)	$ (0.36)