Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest Income
Loans and Leases	$ 252.0	$ 213.4
Securities — Taxable	177.8	143.2
Securities - Non-Taxable	2.0	2.6
Interest-Bearing Due from and Deposits with Banks	19.9	14.9
Federal Reserve and Other Central Bank Deposits	54.2	36.2
Total Interest Income	505.9	410.3
Interest Expense
Deposits	63.1	27.6
Federal Funds Purchased	5.3	0.5
Securities Sold Under Agreements to Repurchase	2.7	0.9
Other Borrowings	26.5	7.6
Senior Notes	11.8	11.7
Long-Term Debt	11.0	7.4
Floating Rate Capital Debt	1.5	1.1
Total Interest Expense	121.9	56.8
Net Interest Income	$ 384.0	$ 353.5