Components of Net Interest Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Interest Income
Loans and Leases	$ 735.9		$ 743.1		$ 828.6
Securities - Taxable	274.9		237.2		250.6
- Non-Taxable	7.2		11.6		17.7
Interest-Bearing Due from and Deposits with Banks	127.6	[1]	142.1	[1]	176.4	[1]
Federal Reserve Deposits and Other	41.3		21.5		14.4
Total Interest Income	1,186.9		1,155.5		1,287.7
Interest Expense
Deposits	81.7		103.3		156.7
Federal Funds Purchased	1.3		1.5		1.2
Securities Sold under Agreements to Repurchase	0.4		0.4		0.4
Other Borrowings	3.4		3.3		4.0
Senior Notes	54.7		74.4		72.0
Long-Term Debt	37.7		37.1		60.3
Floating Rate Capital Debt	2.2		2.4		2.8
Total Interest Expense	181.4		222.4		297.4
Net Interest Income	$ 1,005.5		$ 933.1		$ 990.3

[1] 2014 interest income was earned on cash and due from banks of $1.7 billion and interest-bearing deposits with banks of $14.9 billion as of December 31, 2014.