Maximum Potential Risk, Fair Value, Weighted-Average Years to Maturity, and Underlying Referenced Credit Obligation Type for Credit Default Swaps (Detail) (Corporate Bonds, USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Credit Derivatives [Line Items]
Maximum potential risk	$ (427)	$ (756)
Estimated fair value	(194)	(343)
Less than one year
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
One to three years
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
Three to five years
Credit Derivatives [Line Items]
Maximum potential risk	(147)	(340)
Estimated fair value	(21)	(118)
More than Five and within Ten Years from Balance Sheet Date
Credit Derivatives [Line Items]
Maximum potential risk	(280)	(416)
Estimated fair value	$ (173)	$ (225)