Employee benefits - Reconciliation of net (liabilities) assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension Plans
Reconciliation of net (liabilities) assets:
Net assets/(liabilities) at beginning of year	$ 16,632	$ (8,290)
Amount recognized in AOCL at beginning of year, pre-tax	200,094	243,434
Amount prepaid (liability) at beginning of year	216,726	235,144
Total benefit cost	(12,522)	(18,648)	$ 545
Contributions	230	230
Amount prepaid (liability) at end of year	204,434	216,726	235,144
Amount recognized in AOCL	(177,017)	(200,094)	(243,434)
Net assets/(liabilities) at end of year	27,417	16,632	(8,290)
OPEB Plan
Reconciliation of net (liabilities) assets:
Net assets/(liabilities) at beginning of year	(117,045)	(118,336)
Amount recognized in AOCL at beginning of year, pre-tax	(25,454)	(26,486)
Amount prepaid (liability) at beginning of year	(142,499)	(144,822)
Total benefit cost	(3,620)	(4,061)	(4,476)
Contributions	6,899	6,384
Amount prepaid (liability) at end of year	(139,220)	(142,499)	(144,822)
Amount recognized in AOCL	40,048	25,454	26,486
Net assets/(liabilities) at end of year	$ (99,172)	$ (117,045)	$ (118,336)