Net loss per share attributable to common stockholders - Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net loss attributable to common stockholders, basic and diluted	$ (5,709)	$ (4,819)	$ (22,959)	$ (18,638)
Denominator:
Weighted average common stock, basic and diluted (in shares)	38,189	37,041	37,897	36,722
Net loss per share attributable to common stockholders:
Basic and diluted (in dollars per share)	$ (0.15)	$ (0.13)	$ (0.61)	$ (0.51)