Net loss per share attributable to common stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net loss attributable to common stockholders, basic and diluted	$ (4,819)	$ (3,815)	$ (18,638)	$ (12,997)
Denominator:
Weighted average common stock, basic and diluted (in shares)	37,041	35,881	36,722	35,619
Net loss per share attributable to common stockholders:
Basic and diluted (in dollars per share)	$ (0.13)	$ (0.11)	$ (0.51)	$ (0.36)