Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Loans and Leases
Evaluated on an Individual Basis	$ 106.5		$ 131.7
Evaluated on a Collective Basis	37,300.1		33,628.0
TOTAL LOANS AND LEASES	37,406.6		33,759.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.9		9.1
Evaluated on a Collective Basis	137.9		181.6
Allowance Assigned to Loans and Leases	148.8	$ 165.4	190.7	$ 210.2	$ 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	46.5		59.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	46.5	55.1	61.1	49.0	$ 40.4	19.9
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	195.3		251.8
COMMERCIAL
Loans and Leases
Evaluated on an Individual Basis	56.0		66.6
Evaluated on a Collective Basis	16,151.7		15,195.4
TOTAL LOANS AND LEASES	16,207.7		15,262.0
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.6		8.8
Evaluated on a Collective Basis	107.4		133.4
Allowance Assigned to Loans and Leases	118.0	124.7	142.2	162.8		58.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0		1.6
Evaluated on a Collective Basis	43.6		56.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	43.6	51.7	57.6	46.1		15.8
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	161.6		199.8
PERSONAL
Loans and Leases
Evaluated on an Individual Basis	50.5		65.1
Evaluated on a Collective Basis	21,148.4		18,432.6
TOTAL LOANS AND LEASES	21,198.9		18,497.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	0.3		0.3
Evaluated on a Collective Basis	30.5		48.2
Allowance Assigned to Loans and Leases	30.8	40.7	48.5	47.4		46.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Basis	0.0		0.0
Evaluated on a Collective Basis	2.9		3.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.9	$ 3.4	3.5	$ 2.9		$ 4.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	$ 33.7		$ 52.0