Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Retirement Benefits [Abstract]
Interest cost on benefit obligation	$ 6,150	$ 4,300
Expected return on plan assets	(5,225)	(4,900)
Amortization of prior service credit	(125)	(125)
Amortization of actuarial loss	1,575	1,100
Expense from company-sponsored retirement plans	$ 2,375	$ 375