Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Interest Income
Loans and Leases	$ 239.4	$ 202.1	$ 677.2	$ 604.3
Securities — Taxable	143.4	103.3	421.5	303.4
Securities - Non-Taxable	2.3	2.2	7.5	5.0
Interest-Bearing Due from and Deposits with Banks	16.0	15.4	45.0	49.4
Federal Reserve and Other Central Bank Deposits	52.7	26.2	130.1	83.8
Total Interest Income	453.8	349.2	1,281.3	1,045.9
Interest Expense
Deposits	55.1	20.0	123.7	63.4
Federal Funds Purchased	4.3	0.4	5.7	1.0
Securities Sold Under Agreements to Repurchase	1.8	0.5	4.0	1.5
Other Borrowings	14.2	5.7	31.8	12.4
Senior Notes	11.5	11.8	35.0	35.2
Long-Term Debt	11.3	6.8	28.3	19.3
Floating Rate Capital Debt	1.4	0.9	3.6	2.5
Total Interest Expense	99.6	46.1	232.1	135.3
Net Interest Income	$ 354.2	$ 303.1	$ 1,049.2	$ 910.6