Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Interest Income
Loans and Leases	$ 213.4	$ 200.9
Securities — Taxable	143.2	103.3
Securities - Non-Taxable	2.6	1.1
Interest-Bearing Due from and Deposits with Banks	14.9	17.5
Federal Reserve and Other Central Bank Deposits	36.2	29.2
Total Interest Income	410.3	352.0
Interest Expense
Deposits	27.6	22.2
Federal Funds Purchased	0.5	0.3
Securities Sold Under Agreements to Repurchase	0.9	0.4
Other Borrowings	7.6	2.7
Senior Notes	11.7	11.7
Long-Term Debt	7.4	6.1
Floating Rate Capital Debt	1.1	0.8
Total Interest Expense	56.8	44.2
Net Interest Income	$ 353.5	$ 307.8