Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)			4,225,152
Granted (in shares)			1,538,515
Exercised (in shares)			(239,336)
Cancelled or forfeited (in shares)			(288,783)
Outstanding, ending balance (in shares)	5,235,548		5,235,548
Exercisable (in shares)	2,259,427		2,259,427
Vested and expected to vest (in shares)	5,235,548		5,235,548
Weighted Average Exercise Price
Outstanding (in dollars per share)			$ 15.91
Granted (in dollars per share)			15.50
Exercised (in dollars per share)			10.89
Cancelled or forfeited (in dollars per share)			15.50
Outstanding (in dollars per share)	$ 16.04		16.04
Exercisable (in dollars per share)	14.59		14.59
Vested and expected to vest (in dollars per share)	$ 16.04		$ 16.04
Remaining Contractual Life
Outstanding			8 years 2 months 12 days
Exercisable			7 years 3 months 18 days
Vested and expected to vest			8 years 2 months 12 days
Aggregate Intrinsic Value
Outstanding	$ 16,807		$ 16,807
Exercisable	9,547		9,547
Vested and expected to vest	16,807		16,807
Options disclosures
Stock-based compensation	3,349	$ 2,636	12,120	$ 13,072
Stock options
Options disclosures
Stock-based compensation	$ 3,008	$ 2,559	$ 10,312	$ 12,428
Share-based Payment Arrangement, Employee
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 13.17	$ 12.20	$ 10.29	$ 14.37
Stock-based compensation	$ 3,000	$ 2,500	$ 10,400	$ 12,300
Share-based Payment Arrangement, Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0		0
Share-based Payment Arrangement, Nonemployee | Stock options
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 100	$ 300