OTTI Losses on Available for Sale Securities (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 Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	$ 3		$ 0		$ 7		$ 2
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Total OTTI losses	3	[1]	0	[1]	7	[1]	3	[1]
Portion of losses recognized in OCI (before taxes)	0		0		0		(1)
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	$ 3		$ 0		$ 7		$ 2

[1] The initial OTTI amount represents the excess of the amortized cost over the fair value of AFS debt securities. For subsequent impairments of the same security, amount includes additional declines in the fair value subsequent to the previously recorded OTTI, if applicable, until such time the security is no longer in an unrealized loss position, plus any additional credit losses taken through earnings that exceeds the total impairment.