Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Employee benefit plans
Service cost	$ 1,079	$ 616
Interest cost	71	46
Expected return on plan assets	(50)	(35)
Amortization of prior service cost	45	12
Amortization of actuarial losses	66	34
Net periodic benefit cost	1,211	$ 673
Expected contributions	3,200
Accrued pension obligation	$ 14,000		$ 13,400