Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Banking and Thrift, Interest [Abstract]
Cash and Due from Banks	$ 4,485.9	$ 2,918.9	$ 4,485.9	$ 2,918.9	$ 3,050.6	$ 3,162.4
Interest-Bearing Deposits with Banks	13,935.0		13,935.0		$ 14,928.3
Interest Income
Loans and Leases	186.7	184.8	541.6	553.5
Securities - Taxable	81.5	65.1	235.2	198.0
Securities - Non-Taxable	1.1	1.6	3.7	5.7
Interest-Bearing Due from and Deposits with Banks (1)	27.0	30.9	82.3	96.6
Federal Reserve Deposits and Other	9.4	11.4	30.5	29.2
Total Interest Income	305.7	293.8	893.3	883.0
Interest Expense
Deposits	17.9	21.6	53.0	64.1
Federal Funds Purchased	0.1	0.5	0.4	1.1
Securities Sold Under Agreements to Repurchase	0.0	0.1	0.2	0.3
Other Borrowings	1.0	0.9	3.3	2.4
Senior Notes	11.7	11.6	35.2	42.8
Long-Term Debt	5.5	9.2	18.8	29.0
Floating Rate Capital Debt	0.6	0.6	1.7	1.7
Total Interest Expense	36.8	44.5	112.6	141.4
Net Interest Income	$ 268.9	$ 249.3	$ 780.7	$ 741.6