Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Loans and Leases
Evaluated on an Individual Basis	$ 192.9		$ 131.7
Evaluated on a Collective Basis	39,263.7		33,628.0
TOTAL LOANS AND LEASES	39,456.6		33,759.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.6		9.1
Evaluated on a Collective Basis	133.3		181.6
Allowance Assigned to Loans and Leases	143.9	$ 148.8	190.7	$ 215.4	$ 210.2	$ 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	39.8		59.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	39.8	46.5	61.1	44.9	$ 49.0	19.9
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	183.7		251.8
COMMERCIAL
Loans and Leases
Evaluated on an Individual Basis	116.8		66.6
Evaluated on a Collective Basis	18,329.1		15,195.4
TOTAL LOANS AND LEASES	18,445.9		15,262.0
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	10.6		8.8
Evaluated on a Collective Basis	105.1		133.4
Allowance Assigned to Loans and Leases	115.7	118.0	142.2	147.6		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	37.2		56.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	37.2	43.6	57.6	40.1		15.8
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	152.9		199.8
PERSONAL
Loans and Leases
Evaluated on an Individual Basis	76.1		65.1
Evaluated on a Collective Basis	20,934.6		18,432.6
TOTAL LOANS AND LEASES	21,010.7		18,497.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Basis	0.0		0.3
Evaluated on a Collective Basis	28.2		48.2
Allowance Assigned to Loans and Leases	28.2	30.8	48.5	67.8		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	2.6		3.5
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.6	$ 2.9	3.5	$ 4.8		$ 4.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	$ 30.8		$ 52.0