Retirement plans - Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Comprehensive Income (Loss) [Line Items]
Current year net actuarial (loss) gain	$ 2,387	$ (21,892)	$ 4,463
Amortization of previously deferred prior service costs	5,590	4,158	5,217
Pension and other postretirement benefit items	7,977	(17,434)	10,626
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial (loss) gain	2,356	(23,688)	4,463
Amortization of actuarial loss	6,059	4,583	4,952
Amortization of previously deferred prior service costs	90	90	90
Pension payment timing related charges	0	300	946
Pension and other postretirement benefit items	$ 8,505	$ (18,715)	$ 10,451