Employee benefit plans - Component of Net Periodic Pension Cost that are Included in Accumulated Other Comprehensive Loss (Income) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Loss from plan amendment affecting accumulated postretirement benefit obligation and prior service cost	$ 11,637
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss/(gain)	114,957	$ 109,558
Loss from plan amendment affecting accumulated postretirement benefit obligation and prior service cost	5,557	6,053
Amount in accumulated other comprehensive loss (income)	120,514	115,611
Retiree Health and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss/(gain)	(6,120)	(6,437)
Loss from plan amendment affecting accumulated postretirement benefit obligation and prior service cost	11,637	0
Amount in accumulated other comprehensive loss (income)	$ 5,517	$ (6,437)

Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-14A

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (i-k)
-SubTopic 10
-Topic 220
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

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- Definition

Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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- Definition

Amount, before tax, of accumulated other comprehensive (income) loss for 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).

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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