Reconciliation of Components of Accumulated Other Comprehensive Income Loss (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
Disclosure Reconciliation Of Components Of Accumulated Other Comprehensive Income Loss [Roll Forward]
Balance, beginning of period, Before Tax	$ (306)	$ (177)	$ (287)	$ (291)
Balance, beginning of period, After Tax	(190)	(110)	(178)	(181)
Net change in fair value, Before Tax	4	121	4	338
Net change in fair value, After Tax	3	75	3	210
Gains reclassified to income, Before Tax	(9)	(129)	(28)	(232)
Gains reclassified to income, After Tax	(6)	(80)	(18)	(144)
Balance, end of period, Before Tax	(311)	(185)	(311)	(185)
Balance, end of period, After Tax	$ (193)	$ (115)	$ (193)	$ (115)