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, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Credit Derivatives [Line Items]
Maximum potential risk	$ (381)	$ (756)
Estimated fair value	(130)	(343)
A
Credit Derivatives [Line Items]
Credit Rating	A	A
Maximum potential risk	(146)	(140)
Estimated fair value	(17)	(13)
BB or Lower
Credit Derivatives [Line Items]
Credit Rating	BB or lower	BB or lower
Maximum potential risk	(235)	(616)
Estimated fair value	(113)	(330)
Less than one year
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
Less than one year | A
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
Less than one year | BB or Lower
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
One to three years | A
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
One to three years | BB or Lower
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	(146)	(340)
Estimated fair value	(17)	(118)
Three to five years | A
Credit Derivatives [Line Items]
Maximum potential risk	(146)	(140)
Estimated fair value	(17)	(13)
Three to five years | BB or Lower
Credit Derivatives [Line Items]
Maximum potential risk	0	(200)
Estimated fair value	0	(105)
More than Five and within Ten Years from Balance Sheet Date
Credit Derivatives [Line Items]
Maximum potential risk	(235)	(416)
Estimated fair value	(113)	(225)
More than Five and within Ten Years from Balance Sheet Date | A
Credit Derivatives [Line Items]
Maximum potential risk	0	0
Estimated fair value	0	0
More than Five and within Ten Years from Balance Sheet Date | BB or Lower
Credit Derivatives [Line Items]
Maximum potential risk	(235)	(416)
Estimated fair value	$ (113)	$ (225)