Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income Loss Before Tax [Abstract]
AOCI, beginning balance	$ 506
Unrealized net gain on securities	(113)
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, before Tax	(2)
Unrealized net gain on derivatives	1
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	(114)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	32
AOCI, ending balance	310
Accumulated Other Comprehensive Income Loss Tax [Abstract]
AOCI, beginning balance	(197)
Unrealized net gain on securities	41
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	1
Unrealized net gain on derivatives	(1)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Tax	43
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	(12)
AOCI, ending balance	(125)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI, beginning balance	309
Unrealized net gain on securities	(72)
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Net of Tax	(1)
Unrealized net gain on derivatives	0
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	(71)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	20	(24)
AOCI, ending balance	$ 185