Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Loans
Evaluated on an Individual Basis	$ 109.3	$ 109.1
Evaluated on a Collective Basis	42,260.2	42,784.2
TOTAL LOANS	42,369.5	42,893.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	11.0	10.4
Evaluated on a Collective Basis	148.9	133.9
Allowance Assigned to Loans	159.9	144.3	$ 136.3	$ 138.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	34.3	38.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	34.3	38.5	37.5	34.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	194.2	182.8
COMMERCIAL
Loans
Evaluated on an Individual Basis	63.3	63.0
Evaluated on a Collective Basis	21,546.4	21,572.6
TOTAL LOANS	21,609.7	21,635.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	9.0	10.4
Evaluated on a Collective Basis	121.8	105.8
Allowance Assigned to Loans	130.8	116.2	102.5	105.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	31.3	36.3
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	31.3	36.3	34.7	31.4
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	162.1	152.5
PERSONAL
Loans
Evaluated on an Individual Basis	46.0	46.1
Evaluated on a Collective Basis	20,713.8	21,211.6
TOTAL LOANS	20,759.8	21,257.7
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0	0.0
Evaluated on a Collective Basis	27.1	28.1
Allowance Assigned to Loans	29.1	28.1	33.8	32.8
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	0.0
Evaluated on a Collective Basis	3.0	2.2
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	3.0	2.2	$ 2.8	$ 2.7
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 32.1	$ 30.3