Employee benefits - Reconciliation of net (liabilities) assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension Plans
Reconciliation of net (liabilities) assets:
Net (liabilities) assets at beginning of year	$ 9,013	$ (35,568)
Accumulated other comprehensive loss at beginning of year	225,356	265,899
Amount prepaid (accrued) at beginning of year	234,369	230,331
Net periodic benefit cost	545	3,810	$ (6,165)
Amount prepaid (accrued) at end of year	235,144	234,369	230,331
Accumulated other comprehensive loss	(243,434)	(225,356)	(265,899)
Net (liabilities) assets at end of year	(8,290)	9,013	(35,568)
Contributions	230	228
OPEB Plan
Reconciliation of net (liabilities) assets:
Net (liabilities) assets at beginning of year	(159,958)	(179,210)
Accumulated other comprehensive loss at beginning of year	12,993	32,152
Amount prepaid (accrued) at beginning of year	(146,965)	(147,058)
Net periodic benefit cost	(4,416)	(6,088)	(6,193)
Amount prepaid (accrued) at end of year	(144,822)	(146,965)	(147,058)
Accumulated other comprehensive loss	26,486	(12,993)	(32,152)
Net (liabilities) assets at end of year	(118,336)	(159,958)	$ (179,210)
Contributions	$ 6,619	$ 6,181