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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to common stockholders, basic and diluted	$ 416	$ (522)	$ 2,115	$ (3,229)	$ (1,019)	$ (3,450)	$ (8,017)	$ (6,880)	$ (1,220)	$ (19,366)	$ (27,331)
Denominator:
Weighted average number of common stock, basic and diluted (in shares)									42,066	39,824	38,025
Net loss per share attributable to common stockholders:
Basic and diluted (in dollars per share)	$ 0.01	$ (0.01)	$ 0.05	$ (0.08)	$ (0.02)	$ (0.09)	$ (0.20)	$ (0.18)	$ (0.03)	$ (0.49)	$ (0.72)