Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Loans
Evaluated on an Individual Basis	$ 53.2	$ 96.3
Evaluated on a Collective Basis	47,289.4	47,520.7
TOTAL LOANS	47,342.6	47,617.0
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.5	13.4
Evaluated on a Collective Basis	159.9	165.3
Allowance Assigned to Loans	162.4	178.7	$ 159.9	$ 144.3
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	25.2	26.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	25.2	26.9	34.3	38.5
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	187.6	205.6
COMMERCIAL
Loans
Evaluated on an Individual Basis	14.5	33.7
Evaluated on a Collective Basis	25,402.6	25,379.1
TOTAL LOANS	25,417.1	25,412.8
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	0.5	11.4
Evaluated on a Collective Basis	131.2	135.4
Allowance Assigned to Loans	131.7	146.8	130.8	116.2
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	22.5	24.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	22.5	24.9	31.3	36.3
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	154.2	171.7
PERSONAL
Loans
Evaluated on an Individual Basis	38.7	62.6
Evaluated on a Collective Basis	21,886.8	22,141.6
TOTAL LOANS	21,925.5	22,204.2
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0	2.0
Evaluated on a Collective Basis	28.7	29.9
Allowance Assigned to Loans	30.7	31.9	29.1	28.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on a Collective Basis	2.7	2.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.7	2.0	$ 3.0	$ 2.2
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 33.4	$ 33.9