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, 2012	Mar. 31, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 68.2	$ 94.2
Plus: Losses on Newly Identified Impairments	1.6
Additional Losses on Previously Identified Impairments	1.5	5.1
Less: Current and Prior Period Losses on Securities Sold During the Period	(26.5)
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 44.8	$ 99.3