Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Interest Income
Loans and Leases	$ 202.1	$ 186.7	$ 604.3	$ 541.6
Securities - Taxable	103.3	81.5	303.4	235.2
Securities - Non-Taxable	2.2	1.1	5.0	3.7
Interest-Bearing Due from and Deposits with Banks	15.4	22.1	49.4	64.9
Federal Reserve and Other Central Bank Deposits	26.2	14.3	83.8	47.9
Total Interest Income	349.2	305.7	1,045.9	893.3
Interest Expense
Deposits	20.0	17.9	63.4	53.0
Federal Funds Purchased	0.4	0.1	1.0	0.4
Securities Sold Under Agreements to Repurchase	0.5	0.0	1.5	0.2
Other Borrowings	5.7	1.0	12.4	3.3
Senior Notes	11.8	11.7	35.2	35.2
Long-Term Debt	6.8	5.5	19.3	18.8
Floating Rate Capital Debt	0.9	0.6	2.5	1.7
Total Interest Expense	46.1	36.8	135.3	112.6
Net Interest Income	$ 303.1	$ 268.9	$ 910.6	$ 780.7