Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Interest Income
Loans and Leases	$ 172.2	$ 189.1	$ 344.2	$ 439.3
Securities — Taxable	165.7	208.4	339.6	449.5
Securities - Non-Taxable	0.4	0.3	0.8	0.8
Interest-Bearing Due from and Deposits with Banks	2.7	4.5	5.1	17.3
Federal Reserve and Other Central Bank Deposits and Other	2.1	4.0	3.3	28.6
Total Interest Income	343.1	406.3	693.0	935.5
Interest Expense
Deposits	(14.8)	(0.7)	(28.2)	60.2
Federal Funds Purchased	(0.5)	0.0	(0.4)	2.0
Securities Sold Under Agreements to Repurchase	0.0	0.0	0.0	1.0
Other Borrowings	3.2	6.4	6.7	35.6
Senior Notes	13.8	20.0	27.5	37.9
Long-Term Debt	5.3	7.0	10.6	15.4
Floating Rate Capital Debt	0.5	1.5	1.1	3.2
Total Interest Expense	7.5	34.2	17.3	155.3
Net Interest Income	$ 335.6	$ 372.1	$ 675.7	$ 780.2