Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$ 201.9		$ 242.2
Evaluated for Inherent Impairment	33,176.5		31,398.0
Total Loans and Leases	33,378.4		31,640.2
Specifically Evaluated for Impairment	4.0		6.6
Evaluated for Inherent Impairment	238.2		260.4
Allowance Assigned to Loans and Leases	242.2		267.0
Commitments and Standby Letters of Credit	12.5		28.9
Total Allowance for Credit Losses	254.7	$ 274.2	295.9	$ 298.3	$ 303.5	$ 307.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	48.2		78.9
Evaluated for Inherent Impairment	15,469.2		14,274.7
Total Loans and Leases	15,517.4		14,353.6
Specifically Evaluated for Impairment	3.6		5.8
Evaluated for Inherent Impairment	126.4		138.0
Allowance Assigned to Loans and Leases	130.0		143.8
Commitments and Standby Letters of Credit	9.2		25.9
Total Allowance for Credit Losses	139.2	152.7	169.7	169.1	168.7	168.0
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	153.7		163.3
Evaluated for Inherent Impairment	17,707.3		17,123.3
Total Loans and Leases	17,861.0		17,286.6
Specifically Evaluated for Impairment	0.4		0.8
Evaluated for Inherent Impairment	111.8		122.4
Allowance Assigned to Loans and Leases	112.2		123.2
Commitments and Standby Letters of Credit	3.3		3.0
Total Allowance for Credit Losses	$ 115.5	$ 121.5	$ 126.2	$ 129.2	$ 134.8	$ 139.9