Credit-Related Losses Recognized in Earnings on Debt Securities Other-than-temporarily Impaired (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$ 43.2	$ 116.2	$ 68.2	$ 94.2
Plus: Losses on Newly Identified Impairments			1.6	1.5
Additional Losses on Previously Identified Impairments	0.2	1.3	1.7	21.8
Less: Current and Prior Period Losses on Securities Sold During the Period		(49.3)	(28.1)	(49.3)
Cumulative Credit-Related Losses on Securities Held - End of Period	$ 43.4	$ 68.2	$ 43.4	$ 68.2