Summary of Credit-Related Impairment Losses on Other-Than-Temporarily-Impaired Securities With Bifurcated Losses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Investments [Line Items]
Cumulative credit loss impairments, beginning of period	$ 13	$ 24	$ 0
Credit losses on securities for which an other-than-temporary impairment was not previously recognized	0	0	24
Credit losses for which an other-than-temporary impairment was previously recognized	0	1	0
Securities sold during period	(13)	(12)	0
Cumulative credit loss impairments, end of period	$ 0	$ 13	$ 24