Employee benefit plans - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Defined Benefit Pension Plan
Employer Service cost	$ 1,288	$ 1,032	$ 2,550	$ 2,111
Interest cost	86	68	169	139
Expected return on plan assets	(61)	(47)	(120)	(97)
Amortization of prior service cost	62	45	107	90
Amortization of actuarial losses	46	66	112	132
Net periodic benefit cost	$ 1,421	$ 1,164	$ 2,818	$ 2,375
Location of costs excluding the service component	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Expected contributions	$ 3,500		$ 3,500
Accrued pension obligation	$ 16,200		$ 16,200		$ 15,700