Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Retirement Benefits [Abstract]
Interest cost on benefit obligation	$ 4,300	$ 3,950
Expected return on plan assets	(4,900)	(8,650)
Amortization of prior service cost	(125)	25
Amortization of actuarial loss	1,100	1,200
Expense (income) from company-sponsored retirement plans	$ 375	$ (3,475)