Changes in Level 3 Assets (Detail) (Auction Rate Securities, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets Beginning Balance	$ 98.9		$ 97.8
Total Gains and (Losses):
Included in Earnings			0.1	[1]
Included in Other Comprehensive Income	(0.2)	[2]	2.7	[2]
Purchases, Issuances, Sales, and Settlements:
Sales	0.1
Settlements	(0.3)		(1.0)
Fair Value Assets Ending Balance	$ 98.5		$ 99.6

[1] Realized gains for the three month period ended March 31, 2013 of $0.1 million represent gains from redemptions by issuers, which are recorded in interest income within the consolidated statement of income.

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