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Employee benefit plans - Amounts Recognized in Other Comprehensive Loss/(Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Plans
Adjustments arising during the period:
Net actuarial loss/(gain)	$ 10,709	$ 4,839	$ (63,684)
Prior service cost	430	678	837
Net settlements/curtailments	(1,010)	(522)	(550,706)
Amortization recognized during the period:
Net actuarial (loss)/gain	(4,300)	(6,240)	(16,503)
Prior service (cost)/credit	(926)	(913)	(900)
Total recognized in other comprehensive loss/(income)	4,903	(2,158)	(630,956)
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	22,707	9,081	(57,633)
Retiree Health and Life Insurance Plans
Adjustments arising during the period:
Net actuarial loss/(gain)	(451)	(761)	(412)
Prior service cost	11,637	0	0
Net settlements/curtailments	0	0	0
Amortization recognized during the period:
Net actuarial (loss)/gain	768	681	744
Prior service (cost)/credit	0	0	0
Total recognized in other comprehensive loss/(income)	11,954	(80)	332
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	$ 11,610	$ (622)	$ (285)

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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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