Net Interest Income - Net Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Income
Loans and Leases	$ 1,153.4	$ 1,098.8	$ 919.1
Securities – Taxable	1,070.7	905.2	594.1
Securities - Non-Taxable	3.8	7.0	9.8
Interest-Bearing Due from and Deposits with Banks	72.4	70.0	63.8
Federal Reserve and Other Central Bank Deposits and Other	199.6	240.4	182.6
Total Interest Income	2,499.9	2,321.4	1,769.4
Interest Expense
Deposits	488.9	384.6	182.1
Federal Funds Purchased	25.9	50.3	10.4
Securities Sold under Agreements to Repurchase	6.4	7.8	6.0
Other Borrowings	181.7	150.1	50.7
Senior Notes	72.6	53.4	46.9
Long-Term Debt	38.3	45.0	39.2
Floating Rate Capital Debt	8.2	7.5	4.9
Total Interest Expense	822.0	698.7	340.2
Net Interest Income	$ 1,677.9	$ 1,622.7	$ 1,429.2