Rollforward of Credit Losses Recognized in Earnings Related to 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
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	$ 28	$ 21	$ 25	$ 20
Additions/reductions	3	0	7	2
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows	0	0	(1)	(1)
Ending balance	$ 31	$ 21	$ 31	$ 21