Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	1,871,237
Granted (in shares)	1,043,800
Exercised (in shares)	(70,938)
Cancelled or forfeited (in shares)	(60,246)
Outstanding, ending balance (in shares)	2,783,853
Exerciseable (in shares)	511,643
Vested and expected to vest (in shares)	2,783,853
Weighted Average Exercise Price
Oustanding (in dollars per share)	$ 10.21
Granted (in dollars per share)	11.94
Exercised (in dollars per share)	8.49
Cancelled or forfeited (in dollars per share)	9.93
Oustanding (in dollars per share)	10.91
Exercisable (in dollars per share)	9.30
Vested and expected to vest (in dollars per share)	$ 10.91
Remaining Contractual Life
Outstanding	8 years 10 months 24 days
Exerciseable	8 years
Vested and expected to vest	8 years 10 months 24 days
Aggregate Intrinsic Value
Oustanding	$ 6,858
Exerciseable	2,075
Vested and expected to vest	6,858
Options disclosures
Stock-based compensation	$ 2,165	$ 1,422
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.82
Stock-based compensation	$ 1,300
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)		$ 7.10
Stock-based compensation	$ 200