Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Loans and Leases
Evaluated on an Individual Basis	$ 122.7		$ 166.6
Evaluated on a Collective Basis	41,085.1		40,314.0
TOTAL LOANS AND LEASES	41,207.8		40,480.6
Allowance for Credit Losses on Loans and Leases
Evaluated on an Individual Basis	16.2		10.1
Evaluated on a Collective Basis	122.0		128.3
Allowance Assigned to Loans and Leases	138.2	$ 136.3	138.4	$ 148.8	$ 165.4	$ 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	43.5		34.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	43.5	37.5	34.1	46.5	55.1	61.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	181.7		172.5
COMMERCIAL
Loans and Leases
Evaluated on an Individual Basis	76.0		92.1
Evaluated on a Collective Basis	20,350.8		18,395.3
TOTAL LOANS AND LEASES	20,426.8		18,487.4
Allowance for Credit Losses on Loans and Leases
Evaluated on an Individual Basis	16.2		10.1
Evaluated on a Collective Basis	86.3		95.5
Allowance Assigned to Loans and Leases	102.5	102.5	105.6	118.0	124.7	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	40.3		31.4
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	40.3	34.7	31.4	43.6	51.7	57.6
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	142.8		137.0
PERSONAL
Loans and Leases
Evaluated on an Individual Basis	46.7		74.5
Evaluated on a Collective Basis	20,734.3		21,918.7
TOTAL LOANS AND LEASES	20,781.0		21,993.2
Allowance for Credit Losses on Loans and Leases
Evaluated on an Individual Basis	0.0		0.0
Evaluated on a Collective Basis	35.7		32.8
Allowance Assigned to Loans and Leases	35.7	33.8	32.8	30.8	40.7	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	3.2		2.7
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	3.2	$ 2.8	2.7	$ 2.9	$ 3.4	$ 3.5
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	$ 38.9		$ 35.5