Allowance for Credit Losses (Recorded Investments in Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Loans
Evaluated on an Individual Basis	$ 94.7		$ 109.1
Evaluated on a Collective Basis	43,482.3		42,784.2
TOTAL LOANS	43,577.0		42,893.3
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	10.3		10.4
Evaluated on a Collective Basis	156.5		133.9
Allowance Assigned to Loans	166.8	$ 152.5	144.3	$ 138.7	$ 138.2	$ 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	28.3		38.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	28.3	26.0	38.5	40.7	43.5	34.1
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	195.1		182.8
COMMERCIAL
Loans
Evaluated on an Individual Basis	27.4		63.0
Evaluated on a Collective Basis	21,959.9		21,572.6
TOTAL LOANS	21,987.3		21,635.6
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	8.3		10.4
Evaluated on a Collective Basis	129.1		105.8
Allowance Assigned to Loans	137.4	124.0	116.2	100.6	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	25.5		36.3
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	25.5	23.3	36.3	37.5	40.3	31.4
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	162.9		152.5
PERSONAL
Loans
Evaluated on an Individual Basis	67.3		46.1
Evaluated on a Collective Basis	21,522.4		21,211.6
TOTAL LOANS	21,589.7		21,257.7
Allowance for Credit Losses on Loans
Evaluated on an Individual Basis	2.0		0.0
Evaluated on a Collective Basis	27.4		28.1
Allowance Assigned to Loans	29.4	28.5	28.1	38.1	35.7	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	2.8		2.2
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.8	$ 2.7	2.2	$ 3.2	$ 3.2	$ 2.7
Total Allowance Assigned to Loans and Undrawn Loan Commitments and Standby Letters of Credit	$ 32.2		$ 30.3