Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Loans and Leases
Evaluated on an Individual Basis	$ 137.9	$ 166.6
Evaluated on a Collective Basis	39,101.2	40,314.0
TOTAL LOANS AND LEASES	39,239.1	40,480.6
Allowance for Credit Losses on Loans and Leases
Evaluated on an Individual Basis	16.9	10.1
Evaluated on a Collective Basis	119.4	128.3
Allowance Assigned to Loans and Leases	136.3	138.4	$ 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	37.5	34.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	37.5	34.1	55.1	61.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	173.8	172.5
COMMERCIAL
Loans and Leases
Evaluated on an Individual Basis	76.4	92.1
Evaluated on a Collective Basis	19,148.3	18,395.3
TOTAL LOANS AND LEASES	19,224.7	18,487.4
Allowance for Credit Losses on Loans and Leases
Evaluated on an Individual Basis	16.9	10.1
Evaluated on a Collective Basis	85.6	95.5
Allowance Assigned to Loans and Leases	102.5	105.6	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	34.7	31.4
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	34.7	31.4	51.7	57.6
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	137.2	137.0
PERSONAL
Loans and Leases
Evaluated on an Individual Basis	61.5	74.5
Evaluated on a Collective Basis	19,952.9	21,918.7
TOTAL LOANS AND LEASES	20,014.4	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	33.8	32.8
Allowance Assigned to Loans and Leases	33.8	32.8	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	2.8	2.7
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.8	2.7	$ 3.4	$ 3.5
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	$ 36.6	$ 35.5