Pretax Other-Than-Temporary Impairment Losses by Investment Category (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	$ 166,000,000	$ 13,000,000	$ 1,100,000,000	$ 55,000,000
Perpetual Securities
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	122,000,000	0	306,000,000	41,000,000
Corporate Bonds
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	43,000,000	0	783,000,000	0
Mortgage- and asset-backed Securities
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	1,000,000	12,000,000	9,000,000	12,000,000
Municipalities
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	0	0	1,000,000	0
Equity Securities
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses	$ 0	$ 1,000,000	$ 1,000,000	$ 2,000,000