Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Retirement Benefits [Abstract]
Service cost-benefits earned during the period	$ 0	$ 1	$ 0	$ 1
Interest cost on benefit obligation	4,241	3,988	8,541	7,938
Expected return on plan assets	(4,851)	(8,690)	(9,751)	(17,340)
Amortization of prior service cost	(117)	20	(242)	45
Amortization of actuarial loss	1,202	1,246	2,302	2,446
Expense (income) from company-sponsored retirement plans	$ 475	$ (3,435)	$ 850	$ (6,910)