Other comprehensive income (loss) (reclassification adjustments) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Amounts reclassified from accumulated other comprehensive loss
Prior service credit	$ 355	$ 355	$ 1,066	$ 1,066
Net actuarial loss	(212)	(334)	(636)	(1,001)
Total amortization	143	21	430	65
Tax expense	(74)	(44)	(226)	(133)
Amortization of postretirement benefit plan items, net of tax	69	(23)	204	(68)
Realized gain on cash flow hedges	0	830	0	2,640
Tax expense	0	(223)	0	(712)
Realized gain on cash flow hedges, net of tax	0	607	0	1,928
Total reclassifications, net of tax	$ 69	$ 584	$ 204	$ 1,860

Reclassification adjustments from accumulated other comprehensive income

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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Amount, after tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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