Allowance for Credit Losses (Recorded Investments in Loans and Leases) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Loans and Leases
Specifically Evaluated for Impairment	$ 117.1		$ 116.2
Evaluated for Inherent Impairment	30,727.1		32,373.8
Total Loans and Leases	30,844.2		32,490.0
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	5.5		7.2
Evaluated for Inherent Impairment	100.2		105.4
Allowance Assigned to Loans and Leases	105.7		112.6
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	22.0		25.6
Total Allowance for Credit Losses	127.7	$ 134.1	138.2	$ 140.5	$ 149.2	$ 153.8
Commercial
Loans and Leases
Specifically Evaluated for Impairment	13.5		15.0
Evaluated for Inherent Impairment	13,968.4		15,160.2
Total Loans and Leases	13,981.9		15,175.2
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	5.2		4.1
Evaluated for Inherent Impairment	53.4		53.5
Allowance Assigned to Loans and Leases	58.6		57.6
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	17.8		21.1
Total Allowance for Credit Losses	76.4	78.4	78.7	78.8	81.8	80.8
Personal
Loans and Leases
Specifically Evaluated for Impairment	103.6		101.2
Evaluated for Inherent Impairment	16,758.7		17,213.6
Total Loans and Leases	16,862.3		17,314.8
Allowance for Credit Losses on Credit Exposures
Specifically Evaluated for Impairment	0.3		3.1
Evaluated for Inherent Impairment	46.8		51.9
Allowance Assigned to Loans and Leases	47.1		55.0
Allowance for Undrawn Exposures
Commitments and Standby Letters of Credit	4.2		4.5
Total Allowance for Credit Losses	$ 51.3	$ 55.7	$ 59.5	$ 61.7	$ 67.4	$ 73.0