Changes in Level 3 Assets (Detail) (Auction Rate Securities, USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets Beginning Balance	$ 98.8		$ 99.1		$ 98.9		$ 97.8
Total Gains (Losses):
Included in Earnings	0.7				0.7	[1]	0.1	[1]
Included in Other Comprehensive Income	0.1	[2]	0.3	[2]	0.4	[2]	3.0	[2]
Purchases, Issuances, Sales, and Settlements:
Sales	(14.4)				(14.4)
Settlements					(0.4)		(2.2)
Fair Value Assets Ending Balance	$ 85.2		$ 98.7		$ 85.2		$ 98.7

[1] Realized gains for the three and nine months ended September 30, 2014, of $0.7 million, represent gains from sales of securities. Realized gains for the nine month period ended September 30, 2013, of $0.1 million, represent gains from redemptions by issuers. Gains on sales are recorded in investment security gains (losses) and gains on redemptions are recorded in interest income, within the consolidated statement of income.

[2] Unrealized gains (losses) are included in net unrealized gains (losses) on securities available for sale within the consolidated statement of comprehensive income.