Retirement plans - Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Retirement Benefits [Abstract]
Interest cost on benefit obligation	$ 6,116	$ 4,241	$ 12,266	$ 8,541
Expected return on plan assets	(5,245)	(4,851)	(10,470)	(9,751)
Amortization of prior service credit	(107)	(117)	(232)	(242)
Amortization of actuarial loss	1,434	1,202	3,009	2,302
Expense from company-sponsored retirement plans	$ 2,198	$ 475	$ 4,573	$ 850