Allowance for credit losses - Allowance for loan losses - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Modifications [Line Items]
Outstanding balance of loans modified for borrowers under financial difficulties that were subject to payment default	$ 21.0
Extension of maturity date
Financing Receivable, Modifications [Line Items]
Outstanding balance of loans modified for borrowers under financial difficulties that were subject to payment default	20.0
Combination of reduction of interest rate and extension of maturity
Financing Receivable, Modifications [Line Items]
Outstanding balance of loans modified for borrowers under financial difficulties that were subject to payment default	1.0
Commercial
Financing Receivable, Modifications [Line Items]
Outstanding commitments to lend additional funds to debtors owing receivables whose terms have been modified in troubled debt restructurings	$ 4.2	$ 7.0

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

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Amount of commitment to lend additional funds to debtor experiencing financial difficulty for which creditor has modified terms of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 36
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-36

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