Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loans
Evaluated on an Individual Basis	$ 65.9	$ 96.3
Evaluated on a Collective Basis	43,324.7	47,520.7
TOTAL LOANS	43,390.6	47,617.0
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	3.2	13.4
Evaluated on a Collective Basis	164.8	165.3
Allowance Assigned to Loans	168.0	178.7	$ 144.3	$ 138.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	30.4	26.9	38.5	34.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	198.4	205.6
Commercial
Loans
Evaluated on an Individual Basis	35.9	33.7
Evaluated on a Collective Basis	20,242.9	25,379.1
TOTAL LOANS	20,278.8	25,412.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	1.2	11.4
Evaluated on a Collective Basis	137.3	135.4
Allowance Assigned to Loans	138.5	146.8	116.2	105.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	28.3	24.9	36.3	31.4
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	166.8	171.7
Personal
Loans
Evaluated on an Individual Basis	30.0	62.6
Evaluated on a Collective Basis	23,081.8	22,141.6
TOTAL LOANS	23,111.8	22,204.2
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0	2.0
Evaluated on a Collective Basis	27.5	29.9
Allowance Assigned to Loans	29.5	31.9	28.1	32.8
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	2.1	2.0	$ 2.2	$ 2.7
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 31.6	$ 33.9