Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Interest Income
Loans and Leases	$ 300.6	$ 271.4	$ 598.8	$ 523.4
Securities — Taxable	269.1	219.1	546.5	396.9
Securities - Non-Taxable	1.0	1.9	2.2	3.9
Interest-Bearing Due from and Deposits with Banks	19.2	18.3	37.1	38.2
Federal Reserve and Other Central Bank Deposits and Other	50.3	57.0	118.4	111.2
Total Interest Income	640.2	567.7	1,303.0	1,073.6
Interest Expense
Deposits	133.9	77.3	281.5	140.4
Federal Funds Purchased	7.1	13.6	20.5	18.9
Securities Sold Under Agreements to Repurchase	2.1	2.0	3.9	4.7
Other Borrowings	49.1	35.9	99.0	62.4
Senior Notes	18.5	11.7	34.4	23.5
Long-Term Debt	10.0	12.0	20.0	23.0
Floating Rate Capital Debt	2.1	1.9	4.3	3.4
Total Interest Expense	222.8	154.4	463.6	276.3
Net Interest Income	$ 417.4	$ 413.3	$ 839.4	$ 797.3