Changes in Level 3 Assets (Detail) (Auction Rate Securities, USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets Beginning Balance	$ 99.6		$ 176.3		$ 97.8		$ 178.3
Total Gains and (Losses):
Included in Earnings		[1]	(20.8)	[1]	0.1	[1]	(22.3)	[1]
Included in Other Comprehensive Income		[2]	6.2	[2]	2.7	[2]	6.0	[2]
Purchases, Issues, Sales, and Settlements
Sales			(54.6)				(54.6)
Settlements	(0.5)		(1.9)		(1.5)		(2.2)
Fair Value Assets Ending Balance	$ 99.1		$ 105.2		$ 99.1		$ 105.2

[1] Realized gains for the six months ended June 30, 2013 of $0.1 million represent gains from redemptions by issuers. Realized losses for the three month period ended June 30, 2012 of $20.8 million include $21.1 million of losses from sales of securities partially offset by $0.3 million of gains from redemptions by issuers. Realized losses for the six month period ended June 30, 2012 of $22.3 million include $21.1 million of losses from sales of securities and $1.6 million of impairment losses, partially offset by $0.4 million of gains from redemptions by issuers. Gains on redemptions are recorded in interest income and sales and impairment losses are recorded in investment security gains (losses), within the consolidated statement of income.

[2] Unrealized losses related to auction rate securities are included in net unrealized gains on securities available for sale, within the consolidated statement of comprehensive income.