Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 42.3	$ 68.2
Plus: Losses on Newly Identified Impairments		1.6
Additional Losses on Previously Identified Impairments		1.5
Less: Current and Prior Period Losses on Securities Sold During the Period	(33.5)	(26.5)
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 8.8	$ 44.8