Stock based compensation - Stock Options - (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares) | shares	3,143,566
Granted (in shares) | shares	926,091
Exercised (in shares) | shares	(132,733)
Cancelled or forfeited (in shares) | shares	(128,389)
Outstanding, ending balance (in shares) | shares	3,808,535
Exercisable (in shares) | shares	1,039,408
Vested and expected to vest (in shares) | shares	3,808,535
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 11.82
Granted (in dollars per share)	24.76
Exercised (in dollars per share)	10.64
Cancelled or forfeited (in dollars per share)	14.52
Outstanding (in dollars per share)	14.92
Exercisable (in dollars per share)	10.61
Vested and expected to vest (in dollars per share)	$ 14.92
Remaining Contractual Life
Outstanding	8 years 6 months
Exercisable	7 years 4 months 24 days
Vested and expected to vest	8 years 6 months
Aggregate Intrinsic Value
Outstanding | $	$ 21,020
Exercisable | $	8,548
Vested and expected to vest | $	21,020
Options disclosures
Stock-based compensation | $	$ 2,322	$ 2,165
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 16.49	$ 7.82
Stock-based compensation | $	$ 1,900	$ 1,300