Net Interest Income (Net Interest Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Interest Income
Loans and Leases	$ 181.2	$ 167.1	$ 525.4	$ 606.4
Securities — Taxable	162.9	182.6	502.5	632.1
Securities - Non-Taxable	0.3	0.3	1.1	1.1
Interest-Bearing Due from and Deposits with Banks	1.9	2.7	7.0	20.0
Federal Reserve and Other Central Bank Deposits and Other	5.0	2.7	8.3	31.3
Total Interest Income	351.3	355.4	1,044.3	1,290.9
Interest Expense
Deposits	(16.3)	(3.5)	(44.5)	56.7
Federal Funds Purchased	0.0	0.1	(0.4)	2.1
Securities Sold Under Agreements to Repurchase	0.1	0.0	0.1	1.0
Other Borrowings	3.6	5.0	10.3	40.6
Senior Notes	11.7	19.1	39.2	57.0
Long-Term Debt	5.3	5.7	15.9	21.1
Floating Rate Capital Debt	0.5	0.4	1.6	3.6
Total Interest Expense	4.9	26.8	22.2	182.1
Net Interest Income	$ 346.4	$ 328.6	$ 1,022.1	$ 1,108.8