Net loss per share attributable to common stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss attributable to common stockholders, basic and diluted									$ (22,292)	$ (19,521)	$ (3,968)
Denominator:
Weighted average common stock, basic and diluted (in shares)									36,849	35,753	35,578
Net loss per share attributable to common stockholders:
Basic and diluted (in dollars per share)	$ (0.10)	$ (0.13)	$ (0.16)	$ (0.22)	$ (0.18)	$ (0.11)	$ (0.10)	$ (0.15)	$ (0.60)	$ (0.55)	$ (0.11)