Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Loans
Evaluated on an Individual Basis	$ 100.1		$ 65.9
Evaluated on a Collective Basis	42,849.3		43,324.7
TOTAL LOANS	42,949.4		43,390.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	10.7		3.2
Evaluated on a Collective Basis	153.7		164.8
Allowance Assigned to Loans	164.4	$ 180.5	168.0	$ 184.8	$ 167.7	$ 178.7
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	32.1	34.7	30.4	26.5	29.5	26.9
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	196.5		198.4
COMMERCIAL
Loans
Evaluated on an Individual Basis	52.8		35.9
Evaluated on a Collective Basis	20,518.3		20,242.9
TOTAL LOANS	20,571.1		20,278.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	6.9		1.2
Evaluated on a Collective Basis	127.0		137.3
Allowance Assigned to Loans	133.9	145.8	138.5	152.6	136.9	146.8
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	29.9	32.5	28.3	24.5	27.2	24.9
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	163.8		166.8
PERSONAL
Loans
Evaluated on an Individual Basis	47.3		30.0
Evaluated on a Collective Basis	22,331.0		23,081.8
TOTAL LOANS	22,378.3		23,111.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	3.8		2.0
Evaluated on a Collective Basis	26.7		27.5
Allowance Assigned to Loans	30.5	34.7	29.5	32.2	30.8	31.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	2.2	$ 2.2	2.1	$ 2.0	$ 2.3	$ 2.0
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 32.7		$ 31.6