Other comprehensive income (loss) - Reclassification out of accumulated other comprehensive loss (Details) - Reclassifications Out of Accumulated Other Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Reclassifications Out of Accumulated Other Comprehensive Loss
Net of tax	$ (37,670)	$ (37,469)
Adjustment of pension and postretirement benefit plans
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(2,273)	(3,618)
Income tax benefit	852	1,356
Net of tax	(1,421)	(2,262)
Adjustment of pension and postretirement benefit plans \| Other operating expenses
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(2,273)	(3,618)
Unrealized net holding (losses) gains on debt securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	(45,311)	(44,009)
Income tax benefit	9,062	8,802
Net of tax	(36,249)	(35,207)
Unrealized net holding (losses) gains on debt securities \| Investment securities
Reclassifications Out of Accumulated Other Comprehensive Loss
Before Tax	$ (45,311)	$ (44,009)

Amount before tax of reclassification adjustments of other comprehensive income (loss).

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://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482765/220-10-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481674/830-30-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481694/830-30-45-17

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 20
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481694/830-30-45-20

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477250/944-220-S99-1

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482765/220-10-50-4

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