Employee benefit plans - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Employee benefit plans
Employer Service cost	$ 1,477	$ 1,288	$ 2,934	$ 2,550
Interest cost	47	86	93	169
Expected return on plan assets	(31)	(61)	(61)	(120)
Amortization of prior service cost	53	62	107	107
Amortization of actuarial losses	167	46	334	112
Net periodic benefit cost	$ 1,713	$ 1,421	$ 3,407	$ 2,818
Location of costs excluding the service component	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Expected contributions	$ 4,000		$ 4,000
Accrued pension obligation	$ 25,200		$ 25,200		$ 24,100