Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Loans and Leases
Specifically Evaluated for Impairment	$ 156.1		$ 157.3
Evaluated for Inherent Impairment	34,401.0		33,023.6
Total Loans and Leases	34,557.1		33,180.9
Allowance for Loans and Leases
Specifically Evaluated for Impairment	1.3		3.1
Evaluated for Inherent Impairment	190.7		190.7
Allowance Assigned to Loans and Leases	192.0		193.8
Allowance for Unfunded Exposures
Commitments and Standby Letters of Credit	35.1		39.5
Total Allowance for Credit Losses	227.1	$ 232.6	233.3	$ 274.2	$ 286.7	$ 295.9
Commercial
Loans and Leases
Specifically Evaluated for Impairment	55.3		56.3
Evaluated for Inherent Impairment	16,616.7		15,100.2
Total Loans and Leases	16,672.0		15,156.5
Allowance for Loans and Leases
Specifically Evaluated for Impairment	1.2		3.0
Evaluated for Inherent Impairment	87.5		83.3
Allowance Assigned to Loans and Leases	88.7		86.3
Allowance for Unfunded Exposures
Commitments and Standby Letters of Credit	23.6		28.5
Total Allowance for Credit Losses	112.3	115.2	114.8	152.7	164.3	169.7
Personal
Loans and Leases
Specifically Evaluated for Impairment	100.8		101.0
Evaluated for Inherent Impairment	17,784.3		17,923.4
Total Loans and Leases	17,885.1		18,024.4
Allowance for Loans and Leases
Specifically Evaluated for Impairment	0.1		0.1
Evaluated for Inherent Impairment	103.2		107.4
Allowance Assigned to Loans and Leases	103.3		107.5
Allowance for Unfunded Exposures
Commitments and Standby Letters of Credit	11.5		11.0
Total Allowance for Credit Losses	$ 114.8	$ 117.4	$ 118.5	$ 121.5	$ 122.4	$ 126.2