Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Loans and Leases
Evaluated on an Individual Level	$ 98.5		$ 92.2
Evaluated on a Collective Level	33,658.8		31,317.4
TOTAL LOANS AND LEASES	33,757.3		31,409.6
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	8.7		5.0
Evaluated on a Collective Level	201.5		99.5
Allowance Assigned to Loans and Leases	210.2	$ 147.2	104.5	$ 110.8	$ 114.5	$ 112.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	2.5		1.9
Evaluated on a Collective Level	46.5		18.0
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	49.0	40.4	19.9	23.3	24.9	25.6
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	259.2		124.4
COMMERCIAL
Loans and Leases
Evaluated on an Individual Level	34.3		10.4
Evaluated on a Collective Level	15,735.5		13,990.9
TOTAL LOANS AND LEASES	15,769.8
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	8.6		3.4
Evaluated on a Collective Level	154.2		54.7
Allowance Assigned to Loans and Leases	162.8		58.1
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Evaluated on an Individual Level	2.5		1.9
Evaluated on a Collective Level	43.6		13.9
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	46.1		15.8
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	208.9		73.9
Personal
Loans and Leases
Evaluated on an Individual Level	64.2		81.8
Evaluated on a Collective Level	17,923.3		17,326.5
TOTAL LOANS AND LEASES	17,987.5
Allowance for Credit Losses on Credit Exposures
Evaluated on an Individual Level	0.1		1.6
Evaluated on a Collective Level	47.3		44.8
Allowance Assigned to Loans and Leases	47.4		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	2.9		4.1
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	2.9		4.1
Total Allowance Assigned to Loans and Leases and Undrawn Loan Commitments and Standby Letters of Credit	50.3		50.5
COMMERCIAL
Loans and Leases
TOTAL LOANS AND LEASES	15,769.8		14,001.3
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	162.8	94.7	58.1	59.9	58.1	57.6
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	46.1	38.8	15.8	18.5	20.5	21.1
Personal
Loans and Leases
TOTAL LOANS AND LEASES	17,987.5		17,408.3
Allowance for Credit Losses on Credit Exposures
Allowance Assigned to Loans and Leases	47.4	52.5	46.4	50.9	56.4	55.0
Allowance for Undrawn Loan Commitments and Standby Letters of Credit
Allowance Assigned to Undrawn Loan Commitments and Standby Letters of Credit	$ 2.9	$ 1.6	$ 4.1	$ 4.8	$ 4.4	$ 4.5