Net Interest Income - Net Interest Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Interest Income
Loans and Leases		$ 713.5	$ 774.6	$ 1,153.4
Securities – Taxable		667.6	812.4	1,070.7
Securities - Non-Taxable		1.5	1.4	3.8
Interest-Bearing Due from and Deposits with Banks		9.1	22.4	72.4
Federal Reserve and Other Central Bank Deposits and Other		14.8	32.7	199.6
Total Interest Income	$ 2,499.9	1,406.5	1,643.5	2,499.9
Interest Expense
Deposits		(61.3)	48.4	488.9
Federal Funds Purchased		(0.4)	2.2	25.9
Securities Sold under Agreements to Repurchase		0.2	1.0	6.4
Other Borrowings		14.2	45.3	181.7
Senior Notes		48.3	72.7	72.6
Long-Term Debt		21.1	26.5	38.3
Floating Rate Capital Debt		1.7	4.2	8.2
Total Interest Expense	$ 822.0	23.8	200.3	822.0
Net Interest Income		$ 1,382.7	$ 1,443.2	$ 1,677.9
Low and negative interest rates, percentage of interest-bearing deposits		68.00%