Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Loans and Leases
Evaluated on an Individual Level	$ 98.0		$ 92.2
Evaluated on a Collective Level	32,668.3		31,317.4
TOTAL LOANS AND LEASES	32,766.3		31,409.6
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	6.0		5.0
Evaluated on a Collective Level	209.4		99.5
Allowance Assigned to Loans and Leases	215.4	$ 210.2	104.5	$ 105.7	$ 110.8	$ 112.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	2.2		1.9
Evaluated on a Collective Level	42.7		18.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	44.9	49.0	19.9	22.0	23.3	25.6
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	260.3		124.4
COMMERCIAL
Loans and Leases
Evaluated on an Individual Level	34.1		10.4
Evaluated on a Collective Level	14,547.6		13,990.9
TOTAL LOANS AND LEASES	14,581.7
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	5.6		3.4
Evaluated on a Collective Level	142.0		54.7
Allowance Assigned to Loans and Leases	147.6		58.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	2.2		1.9
Evaluated on a Collective Level	37.9		13.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	40.1		15.8
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	187.7		73.9
Personal
Loans and Leases
Evaluated on an Individual Level	63.9		81.8
Evaluated on a Collective Level	18,120.7		17,326.5
TOTAL LOANS AND LEASES	18,184.6
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	0.4		1.6
Evaluated on a Collective Level	67.4		44.8
Allowance Assigned to Loans and Leases	67.8		46.4
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	0.0		0.0
Evaluated on a Collective Level	4.8		4.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	4.8		4.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	72.6		50.5
COMMERCIAL
Loans and Leases
TOTAL LOANS AND LEASES	14,581.7		14,001.3
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	147.6	162.8	58.1	58.6	59.9	57.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	40.1	46.1	15.8	17.8	18.5	21.1
Personal
Loans and Leases
TOTAL LOANS AND LEASES	18,184.6		17,408.3
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	67.8	47.4	46.4	47.1	50.9	55.0
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	$ 4.8	$ 2.9	$ 4.1	$ 4.2	$ 4.8	$ 4.5