Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Loans
Evaluated on an Individual Basis	$ 73.4		$ 96.3
Evaluated on a Collective Basis	41,876.9		47,520.7
TOTAL LOANS	41,950.3		47,617.0
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	5.5		13.4
Evaluated on a Collective Basis	179.3		165.3
Allowance Assigned to Loans	184.8	$ 167.7	178.7	$ 166.8	$ 152.5	$ 144.3
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	26.5	29.5	26.9	28.3	26.0	38.5
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	211.3		205.6
COMMERCIAL
Loans
Evaluated on an Individual Basis	38.1		33.7
Evaluated on a Collective Basis	20,795.7		25,379.1
TOTAL LOANS	20,833.8		25,412.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	3.3		11.4
Evaluated on a Collective Basis	149.3		135.4
Allowance Assigned to Loans	152.6	136.9	146.8	137.4	124.0	116.2
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	24.5	27.2	24.9	25.5	23.3	36.3
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	177.1		171.7
PERSONAL
Loans
Evaluated on an Individual Basis	35.3		62.6
Evaluated on a Collective Basis	21,081.2		22,141.6
TOTAL LOANS	21,116.5		22,204.2
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.2		2.0
Evaluated on a Collective Basis	30.0		29.9
Allowance Assigned to Loans	32.2	30.8	31.9	29.4	28.5	28.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit - Evaluated on a Collective Basis	2.0	$ 2.3	2.0	$ 2.8	$ 2.7	$ 2.2
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 34.2		$ 33.9