Net Interest Income - Net Interest Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest Income
Loans and Leases		$ 774.6	$ 1,153.4	$ 1,098.8
Securities – Taxable		812.4	1,070.7	905.2
Securities - Non-Taxable		1.4	3.8	7.0
Interest-Bearing Due from and Deposits with Banks		22.4	72.4	70.0
Federal Reserve and Other Central Bank Deposits and Other		32.7	199.6	240.4
Total Interest Income	$ 2,321.4	1,643.5	2,499.9	2,321.4
Interest Expense
Deposits		48.4	488.9	384.6
Federal Funds Purchased		2.2	25.9	50.3
Securities Sold under Agreements to Repurchase		1.0	6.4	7.8
Other Borrowings		45.3	181.7	150.1
Senior Notes		72.7	72.6	53.4
Long-Term Debt		26.5	38.3	45.0
Floating Rate Capital Debt		4.2	8.2	7.5
Total Interest Expense	$ 698.7	200.3	822.0	698.7
Net Interest Income		$ 1,443.2	$ 1,677.9	$ 1,622.7