Components of Net Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest Income
Loans and Leases	$ 806.5	$ 731.9	$ 735.9
Securities – Taxable	428.8	332.2	274.9
Securities - Non-Taxable	7.5	4.8	7.2
Interest-Bearing Due from and Deposits with Banks	64.3	84.9	97.2
Federal Reserve and Other Central Bank Deposits and Other	109.8	70.2	71.7
Total Interest Income	1,416.9	1,224.0	1,186.9
Interest Expense
Deposits	83.5	74.3	81.7
Federal Funds Purchased	1.5	0.7	1.3
Securities Sold under Agreements to Repurchase	2.3	0.3	0.4
Other Borrowings	18.0	5.0	3.4
Senior Notes	46.8	46.8	54.7
Long-Term Debt	26.4	24.4	37.7
Floating Rate Capital Debt	3.5	2.4	2.2
Total Interest Expense	182.0	153.9	181.4
Net Interest Income	$ 1,234.9	$ 1,070.1	$ 1,005.5