Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Loans and Leases
Evaluated on an Individual Basis	$ 123.8	$ 131.7
Evaluated on a Collective Basis	34,220.5	33,628.0
TOTAL LOANS AND LEASES	34,344.3	33,759.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.9	9.1
Evaluated on a Collective Basis	154.5	181.6
Allowance Assigned to Loans and Leases	165.4	190.7	$ 147.2	$ 104.5
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	1.6
Evaluated on a Collective Basis	55.1	59.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	55.1	61.1	40.4	19.9
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	220.5	251.8
COMMERCIAL
Loans and Leases
Evaluated on an Individual Basis	61.4	66.6
Evaluated on a Collective Basis	15,786.5	15,195.4
TOTAL LOANS AND LEASES	15,847.9
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.6	8.8
Evaluated on a Collective Basis	114.1	133.4
Allowance Assigned to Loans and Leases	124.7	142.2
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0	1.6
Evaluated on a Collective Basis	51.7	56.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	51.7	57.6
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	176.4	199.8
Personal
Loans and Leases
Evaluated on an Individual Basis	62.4	65.1
Evaluated on a Collective Basis	18,434.0	18,432.6
TOTAL LOANS AND LEASES	18,496.4
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	0.3	0.3
Evaluated on a Collective Basis	40.4	48.2
Allowance Assigned to Loans and Leases	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	3.4	3.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	3.4	3.5
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	44.1	52.0
COMMERCIAL
Loans and Leases
TOTAL LOANS AND LEASES	15,847.9	15,262.0
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	124.7	142.2	94.7	58.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	51.7	57.6	38.8	15.8
Personal
Loans and Leases
TOTAL LOANS AND LEASES	18,496.4	18,497.7
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	40.7	48.5	52.5	46.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	$ 3.4	$ 3.5	$ 1.6	$ 4.1