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Employee benefit plans - Amounts Recognized in Other Comprehensive Loss/(Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Retirement Plans
Adjustments arising during the period:
Net actuarial (gain)/loss	$ (238)	$ 10,709	$ 4,839
Prior service cost	327	430	678
Net settlements/curtailments	(530)	(1,010)	(522)
Amortization recognized during the period:
Net actuarial (loss)/gain	(4,472)	(4,300)	(6,240)
Prior service cost	(864)	(926)	(913)
Total recognized in other comprehensive (income)/loss	(5,777)	4,903	(2,158)
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	11,509	22,707	9,081
Retiree Health and Life Insurance Plans
Adjustments arising during the period:
Net actuarial (gain)/loss	(2,485)	(451)	(761)
Prior service cost	0	11,637	0
Net settlements/curtailments	0	0	0
Amortization recognized during the period:
Net actuarial (loss)/gain	899	768	681
Prior service cost	(385)	0	0
Total recognized in other comprehensive (income)/loss	(1,971)	11,954	(80)
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	$ (1,780)	$ 11,610	$ (622)

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- Definition

Projected defined benefit plan amortization of gains losses.

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Amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss.

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Amount, before tax, after reclassification adjustment, of increase (decrease) in accumulated other comprehensive income from gain (loss) of defined benefit plan.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from settlement and curtailment.

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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