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
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss attributable to common stockholders	$ (8,099)	$ (7,139)	[1]
Denominator:
Weighted-average common shares outstanding - basic and diluted	42,762	38,407
Net loss per common share - basic and diluted	$ (0.19)	$ (0.19)

[1] See Note 1 for a summary of adjustments