Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Loans
Evaluated on an Individual Basis	$ 45.3		$ 96.3
Evaluated on a Collective Basis	42,089.9		47,520.7
TOTAL LOANS	42,135.2		47,617.0
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.5		13.4
Evaluated on a Collective Basis	165.2		165.3
Allowance Assigned to Loans	167.7	$ 162.4	178.7	$ 152.5	$ 159.9	$ 144.3
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	29.5		26.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	29.5	25.2	26.9	26.0	34.3	38.5
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	197.2		205.6
COMMERCIAL
Loans
Evaluated on an Individual Basis	14.5		33.7
Evaluated on a Collective Basis	20,819.5		25,379.1
TOTAL LOANS	20,834.0		25,412.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	0.5		11.4
Evaluated on a Collective Basis	136.4		135.4
Allowance Assigned to Loans	136.9	131.7	146.8	124.0	130.8	116.2
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	27.2		24.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	27.2	22.5	24.9	23.3	31.3	36.3
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	164.1		171.7
PERSONAL
Loans
Evaluated on an Individual Basis	30.8		62.6
Evaluated on a Collective Basis	21,270.4		22,141.6
TOTAL LOANS	21,301.2		22,204.2
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0		2.0
Evaluated on a Collective Basis	28.8		29.9
Allowance Assigned to Loans	30.8	30.7	31.9	28.5	29.1	28.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	2.3		2.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.3	$ 2.7	2.0	$ 2.7	$ 3.0	$ 2.2
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 33.1		$ 33.9