Net Interest Income - Net Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Income
Loans and Leases	$ 919.1	$ 806.5	$ 731.9
Securities – Taxable	594.1	428.8	332.2
Securities - Non-Taxable	9.8	7.5	4.8
Interest-Bearing Due from and Deposits with Banks	63.8	64.3	84.9
Federal Reserve and Other Central Bank Deposits and Other	182.6	109.8	70.2
Total Interest Income	1,769.4	1,416.9	1,224.0
Interest Expense
Deposits	182.1	83.5	74.3
Federal Funds Purchased	10.4	1.5	0.7
Securities Sold under Agreements to Repurchase	6.0	2.3	0.3
Other Borrowings	50.7	18.0	5.0
Senior Notes	46.9	46.8	46.8
Long-Term Debt	39.2	26.4	24.4
Floating Rate Capital Debt	4.9	3.5	2.4
Total Interest Expense	340.2	182.0	153.9
Net Interest Income	$ 1,429.2	$ 1,234.9	$ 1,070.1