Employee benefits - Reconciliation of net (liabilities) assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Plans
Reconciliation of net (liabilities) assets:
Net (liabilities) assets at beginning of year	$ (8,290)	$ 9,013
Amount recognized in AOCL at beginning of year	243,434	225,356
Amount prepaid (accrued) at beginning of year	235,144	234,369
Total cost (benefit)	18,648	(545)	$ (3,810)
Contributions	230	230
Amount prepaid (accrued) at end of year	216,726	235,144	234,369
Amount recognized in AOCL	(200,094)	(243,434)	(225,356)
Net (liabilities) assets at end of year	16,632	(8,290)	9,013
OPEB Plan
Reconciliation of net (liabilities) assets:
Net (liabilities) assets at beginning of year	(118,336)	(159,958)
Amount recognized in AOCL at beginning of year	(26,486)	12,993
Amount prepaid (accrued) at beginning of year	(144,822)	(146,965)
Total cost (benefit)	4,061	4,476	6,088
Contributions	6,384	6,619
Amount prepaid (accrued) at end of year	(142,499)	(144,822)	(146,965)
Amount recognized in AOCL	25,454	26,486	(12,993)
Net (liabilities) assets at end of year	$ (117,045)	$ (118,336)	$ (159,958)