Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Employee benefit plans
Service cost	$ 1,041	$ 760	$ 3,152	$ 2,056
Interest cost	69	57	208	155
Expected return on plan assets	(48)	(43)	(145)	(116)
Amortization of prior service cost	45	12	135	37
Amortization of actuarial losses	66	34	198	102
Net periodic benefit cost	1,173	$ 820	3,548	$ 2,234
Expected contributions	3,200		3,200
Accrued pension obligation	$ 14,000		$ 14,000		$ 13,400