Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)			3,143,566
Granted (in shares)			2,030,191
Exercised (in shares)			(360,099)
Cancelled or forfeited (in shares)			(647,172)
Outstanding, ending balance (in shares)	4,166,486		4,166,486
Exercisable (in shares)	1,375,760		1,375,760
Vested and expected to vest (in shares)	4,166,486		4,166,486
Weighted Average Exercise Price
Outstanding (in dollars per share)			$ 11.82
Granted (in dollars per share)			21.54
Exercised (in dollars per share)			10.17
Cancelled or forfeited (in dollars per share)			16.20
Outstanding (in dollars per share)	$ 16.02		16.02
Exercisable (in dollars per share)	12.02		12.02
Vested and expected to vest (in dollars per share)	$ 16.02		$ 16.02
Remaining Contractual Life
Outstanding			8 years 7 months 6 days
Exercisable			7 years 6 months
Vested and expected to vest			8 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding	$ 17,613		$ 17,613
Exercisable	10,288		10,288
Vested and expected to vest	17,613		17,613
Options disclosures
Stock-based compensation	$ 2,636	$ 3,241	$ 13,072	$ 6,945
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 12.20	$ 9.43	$ 14.37	$ 7.88
Stock-based compensation	$ 2,500	$ 1,400	$ 12,300	$ 3,800
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0	20,000	0	20,000
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 300	$ 300