Credit-Related Losses Recognized in Earnings on Debt Securities Other-than-temporarily Impaired (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 44.8	$ 99.3	$ 68.2	$ 94.2
Plus: Losses on Newly Identified Impairments		1.5	1.6	1.5
Additional Losses on Previously Identified Impairments		15.4	1.5	20.5
Less: Current and Prior Period Losses on Securities Sold During the Period	(1.6)		(28.1)
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 43.2	$ 116.2	$ 43.2	$ 116.2