Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Loans
Evaluated on an Individual Basis	$ 77.9		$ 109.1
Evaluated on a Collective Basis	43,468.8		42,784.2
TOTAL LOANS	43,546.7		42,893.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	11.0		10.4
Evaluated on a Collective Basis	141.5		133.9
Allowance Assigned to Loans	152.5	$ 159.9	144.3	$ 138.2	$ 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	26.0		38.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	26.0	34.3	38.5	43.5	37.5	34.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	178.5		182.8
COMMERCIAL
Loans
Evaluated on an Individual Basis	26.7		63.0
Evaluated on a Collective Basis	22,334.5		21,572.6
TOTAL LOANS	22,361.2		21,635.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	9.0		10.4
Evaluated on a Collective Basis	115.0		105.8
Allowance Assigned to Loans	124.0	130.8	116.2	102.5	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	23.3		36.3
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	23.3	31.3	36.3	40.3	34.7	31.4
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	147.3		152.5
PERSONAL
Loans
Evaluated on an Individual Basis	51.2		46.1
Evaluated on a Collective Basis	21,134.3		21,211.6
TOTAL LOANS	21,185.5		21,257.7
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0		0.0
Evaluated on a Collective Basis	26.5		28.1
Allowance Assigned to Loans	28.5	29.1	28.1	35.7	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	2.7		2.2
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.7	$ 3.0	2.2	$ 3.2	$ 2.8	$ 2.7
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 31.2		$ 30.3