Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Loans
Evaluated on an Individual Basis	$ 79.6	$ 100.9
Evaluated on a Collective Basis	42,425.9	41,847.4
TOTAL LOANS	42,505.5	41,948.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	13.7	10.2
Evaluated on a Collective Basis	147.4	154.1
Allowance Assigned to Loans	161.1	164.3	$ 167.1	$ 168.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	25.5	23.3	32.8	30.4
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	186.6	187.6
COMMERCIAL
Loans
Evaluated on an Individual Basis	38.4	55.0
Evaluated on a Collective Basis	20,640.4	20,376.0
TOTAL LOANS	20,678.8	20,431.0
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	9.4	5.9
Evaluated on a Collective Basis	122.1	126.6
Allowance Assigned to Loans	131.5	132.5	133.6	138.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	21.5	21.7	30.7	28.3
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	153.0	154.2
PERSONAL
Loans
Evaluated on an Individual Basis	41.2	45.9
Evaluated on a Collective Basis	21,785.5	21,471.4
TOTAL LOANS	21,826.7	21,517.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	4.3	4.3
Evaluated on a Collective Basis	25.3	27.5
Allowance Assigned to Loans	29.6	31.8	33.5	29.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	4.0	1.6	$ 2.1	$ 2.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 33.6	$ 33.4