Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest Income
Loans and Leases	$ 282.9	$ 239.4	$ 806.3	$ 677.2
Securities — Taxable	238.8	143.4	635.8	421.5
Securities - Non-Taxable	1.7	2.3	5.5	7.5
Interest-Bearing Due from and Deposits with Banks	15.6	16.0	53.8	45.0
Federal Reserve and Other Central Bank Deposits	60.2	52.7	171.4	130.1
Total Interest Income	599.2	453.8	1,672.8	1,281.3
Interest Expense
Deposits	105.4	55.1	245.8	123.7
Federal Funds Purchased	16.1	4.3	35.0	5.7
Securities Sold Under Agreements to Repurchase	1.5	1.8	6.1	4.0
Other Borrowings	40.1	14.2	102.6	31.8
Senior Notes	14.6	11.5	38.1	35.0
Long-Term Debt	11.3	11.3	34.3	28.3
Floating Rate Capital Debt	2.0	1.4	5.4	3.6
Total Interest Expense	191.0	99.6	467.3	232.1
Net Interest Income	$ 408.2	$ 354.2	$ 1,205.5	$ 1,049.2