Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 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)			1,541,000
Exercised (in shares)			(130,129)
Cancelled or forfeited (in shares)			(237,494)
Outstanding, ending balance (in shares)	3,044,614		3,044,614
Exercisable (in shares)	827,081		827,081
Vested and expected to vest (in shares)	3,044,614		3,044,614
Weighted Average Exercise Price
Outstanding (in dollars per share)			$ 10.21
Granted (in dollars per share)			12.16
Exercised (in dollars per share)			8.42
Cancelled or forfeited (in dollars per share)			10.63
Outstanding (in dollars per share)	$ 11.23		11.23
Exercisable (in dollars per share)	10.34		10.34
Vested and expected to vest (in dollars per share)	$ 11.23		$ 11.23
Remaining Contractual Life
Outstanding			8 years 9 months 18 days
Exercisable			8 years 2 months 12 days
Vested and expected to vest			8 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding	$ 28,542		$ 28,542
Exercisable	8,499		8,499
Vested and expected to vest	28,542		28,542
Options disclosures
Stock-based compensation	$ 3,241	$ 1,789	$ 6,945	$ 4,528
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 9.43	$ 8.98	$ 7.88	$ 7.62
Stock-based compensation	$ 1,400	$ 900	$ 3,800	$ 2,200
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)			$ 6.88	$ 9.21
Stock-based compensation	$ 100	$ 100	$ 300	$ 100