Net Interest Income - Net Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest Income
Loans and Leases	$ 1,098.8	$ 919.1	$ 806.5
Securities – Taxable	905.2	594.1	428.8
Securities - Non-Taxable	7.0	9.8	7.5
Interest-Bearing Due from and Deposits with Banks	70.0	63.8	64.3
Federal Reserve and Other Central Bank Deposits and Other	240.4	182.6	109.8
Total Interest Income	2,321.4	1,769.4	1,416.9
Interest Expense
Deposits	384.6	182.1	83.5
Federal Funds Purchased	50.3	10.4	1.5
Securities Sold under Agreements to Repurchase	7.8	6.0	2.3
Other Borrowings	150.1	50.7	18.0
Senior Notes	53.4	46.9	46.8
Long-Term Debt	45.0	39.2	26.4
Floating Rate Capital Debt	7.5	4.9	3.5
Total Interest Expense	698.7	340.2	182.0
Net Interest Income	$ 1,622.7	$ 1,429.2	$ 1,234.9