Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Loans
Evaluated on an Individual Basis	$ 79.3	$ 65.9
Evaluated on a Collective Basis	40,754.0	43,324.7
TOTAL LOANS	40,833.3	43,390.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	3.0	3.2
Evaluated on a Collective Basis	164.1	164.8
Allowance Assigned to Loans	167.1	168.0	$ 162.4	$ 178.7
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	32.8	30.4	25.2	26.9
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	199.9	198.4
COMMERCIAL
Loans
Evaluated on an Individual Basis	40.4	35.9
Evaluated on a Collective Basis	19,442.1	20,242.9
TOTAL LOANS	19,482.5	20,278.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	0.7	1.2
Evaluated on a Collective Basis	132.9	137.3
Allowance Assigned to Loans	133.6	138.5	131.7	146.8
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	30.7	28.3	22.5	24.9
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	164.3	166.8
PERSONAL
Loans
Evaluated on an Individual Basis	38.9	30.0
Evaluated on a Collective Basis	21,311.9	23,081.8
TOTAL LOANS	21,350.8	23,111.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.3	2.0
Evaluated on a Collective Basis	31.2	27.5
Allowance Assigned to Loans	33.5	29.5	30.7	31.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	2.1	2.1	$ 2.7	$ 2.0
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 35.6	$ 31.6