Components of Net Interest Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest Income
Loans and Leases	$ 731.9	$ 735.9	$ 743.1
Securities – Taxable	332.2	274.9	237.2
Securities - Non-Taxable	4.8	7.2	11.6
Interest-Bearing Due from and Deposits with Banks	108.3	127.6	142.1
Federal Reserve Deposits and Other	46.8	41.3	21.5
Total Interest Income	1,224.0	1,186.9	1,155.5
Interest Expense
Deposits	74.3	81.7	103.3
Federal Funds Purchased	0.7	1.3	1.5
Securities Sold under Agreements to Repurchase	0.3	0.4	0.4
Other Borrowings	5.0	3.4	3.3
Senior Notes	46.8	54.7	74.4
Long-Term Debt	24.4	37.7	37.1
Floating Rate Capital Debt	2.4	2.2	2.4
Total Interest Expense	153.9	181.4	222.4
Net Interest Income	$ 1,070.1	$ 1,005.5	$ 933.1