Loans - Loan Categories Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Schedule of Loans by Loan Classification [Line Items]
Loan origination fees	$ 4.1	$ 5.4
Accrued interest receivable on loans	$ 17.8	$ 16.1
Accrued interest, statement of financial position	Accrued interest receivable	Accrued interest receivable
Outstanding loan balances to related parties	$ 35.4	$ 31.4
Unadvanced lines of credit available to related parties	34.8	35.7
New loans and net increases to loan balances to related parties during period	10.5	1.5
Principal paydowns on related party loans	7.4	19.7
Commercial
Schedule of Loans by Loan Classification [Line Items]
Participation loans amount	163.7	126.6
Participations loans sold that are still serviced amount	77.4	69.8
Tax exempt interest income on qualified commercial loans	2.2	2.0	$ 1.7
Average tax exempt loan balances	47.9	47.0
Residential mortgages
Schedule of Loans by Loan Classification [Line Items]
Amount of loans serviced for others	$ 6.7	$ 7.7

Average Loan Balance During Period of Tax Exempt Loans

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Interest Income, Commercial Loans, Tax Exempt

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Schedule of Loans by Loan Classification [Line Items]

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Amount, after allowance for credit loss, of accrued interest on financing receivable.

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Indicates line item in statement of financial position that includes accrued interest, after allowance for credit loss, on financing receivable.

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Amount of unamortized loan commitment, origination, and other costs (fees) on financing receivable recognized as adjustment to yield. Excludes financing receivable covered under loss sharing agreement.

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For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

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Amount of cash inflow from related parties which reduce the amount of loans and leases receivable due from related parties.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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