Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Interest Income
Loans and Leases	$ 224.4	$ 201.3	$ 437.8	$ 402.2
Securities — Taxable	135.0	96.9	278.2	200.1
Securities - Non-Taxable	2.5	1.6	5.1	2.8
Interest-Bearing Due from and Deposits with Banks	14.1	16.5	29.0	34.0
Federal Reserve and Other Central Bank Deposits	41.2	28.4	77.4	57.6
Total Interest Income	417.2	344.7	827.5	696.7
Interest Expense
Deposits	41.0	21.2	68.6	43.4
Federal Funds Purchased	0.9	0.3	1.4	0.6
Securities Sold Under Agreements to Repurchase	1.3	0.6	2.2	1.0
Other Borrowings	10.0	4.0	17.6	6.7
Senior Notes	11.8	11.7	23.5	23.4
Long-Term Debt	9.6	6.4	17.0	12.5
Floating Rate Capital Debt	1.1	0.8	2.2	1.6
Total Interest Expense	75.7	45.0	132.5	89.2
Net Interest Income	$ 341.5	$ 299.7	$ 695.0	$ 607.5