Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest Income
Loans and Leases	$ 271.4	$ 224.4	$ 523.4	$ 437.8
Securities — Taxable	219.1	135.0	396.9	278.2
Securities - Non-Taxable	1.9	2.5	3.9	5.1
Interest-Bearing Due from and Deposits with Banks	18.3	14.1	38.2	29.0
Federal Reserve and Other Central Bank Deposits	57.0	41.2	111.2	77.4
Total Interest Income	567.7	417.2	1,073.6	827.5
Interest Expense
Deposits	77.3	41.0	140.4	68.6
Federal Funds Purchased	13.6	0.9	18.9	1.4
Securities Sold Under Agreements to Repurchase	2.0	1.3	4.7	2.2
Other Borrowings	35.9	10.0	62.4	17.6
Senior Notes	11.7	11.8	23.5	23.5
Long-Term Debt	12.0	9.6	23.0	17.0
Floating Rate Capital Debt	1.9	1.1	3.4	2.2
Total Interest Expense	154.4	75.7	276.3	132.5
Net Interest Income	$ 413.3	$ 341.5	$ 797.3	$ 695.0