Employee benefit plans - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Employee benefit plans
Employer Service cost	$ 1,457	$ 1,263
Interest cost	46	84
Expected return on plan assets	(30)	(60)
Amortization of prior service cost	54	45
Amortization of actuarial losses	167	66
Net periodic benefit cost	$ 1,694	$ 1,398
Location of costs excluding the service component	us-gaap:OtherNonoperatingIncomeExpense	us-gaap:OtherNonoperatingIncomeExpense
Expected contributions	$ 4,000
Accrued pension obligation	$ 24,600		$ 24,100