Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Loans
Evaluated on an Individual Basis	$ 96.3	$ 109.1
Evaluated on a Collective Basis	47,520.7	42,784.2
TOTAL LOANS	47,617.0	42,893.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	13.4	10.4
Evaluated on a Collective Basis	165.3	133.9
Allowance Assigned to Loans	178.7	144.3	$ 138.4	$ 190.7
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	26.9	38.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	26.9	38.5	34.1	61.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	205.6	182.8
Commercial
Loans
Evaluated on an Individual Basis	33.7	63.0
Evaluated on a Collective Basis	25,379.1	21,572.6
TOTAL LOANS	25,412.8	21,635.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	11.4	10.4
Evaluated on a Collective Basis	135.4	105.8
Allowance Assigned to Loans	146.8	116.2	105.6	142.2
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	24.9	36.3
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	24.9	36.3	31.4	57.6
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	171.7	152.5
PERSONAL
Loans
Evaluated on an Individual Basis	62.6	46.1
Evaluated on a Collective Basis	22,141.6	21,211.6
TOTAL LOANS	22,204.2	21,257.7
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0	0.0
Evaluated on a Collective Basis	29.9	28.1
Allowance Assigned to Loans	31.9	28.1	32.8	48.5
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	2.0	2.2
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.0	2.2	$ 2.7	$ 3.5
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 33.9	$ 30.3