Activity of the Components of Other Comprehensive Income and Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial loss (gain)	$ 7	$ 50	$ 2
Other Comprehensive Income Loss Before Tax Period Increase Decrease	(145)	241	112
Total	50	(85)	(39)
Total Accumulated Other Comprehensive Income - Beginning Balance	470	314	241
Unrealized holding gains (losses) on available-for-sale securities arising during period	(63)	201	143
Other Comprehensive Income Loss Reclassification Adjustment For Sale Of Securities Included In Net Income Net Of Tax	(10)	(37)	(38)
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	24	58	1
Other Comprehensive Income Loss Reclassification Adjustment On Derivatives Included In Net Income Net Of Tax	(54)	(45)	(39)
Net actuarial loss (gain)	8	(21)	6
Other Comprehensive Income (Loss) Net Of Tax	(95)	156	73
Total Accumulated Other Comprehensive Income - Ending Balance	375	470	314
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains on available-for-sale securities arising during period	(97)	309	216
Other Comprehensive Income Reclassification Adjustment For Sale Of Securities Included In Net Income Before Tax	(15)	(56)	(57)
Other Comprehensive Income Loss Available For Sale Securities Adjustment Before Tax	(112)	253	159
Unrealized holding gains on available-for-sale securities arising during period	34	(108)	(73)
Other Comprehensive Income Loss Reclassification Adjustment For Sale Of Securities Included In Net Income Tax	(5)	(19)	(19)
OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax	39	(89)	(54)
Total Accumulated Other Comprehensive Income - Beginning Balance	485	321	216
Unrealized holding gains (losses) on available-for-sale securities arising during period	(63)	201	143
Other Comprehensive Income Loss Reclassification Adjustment For Sale Of Securities Included In Net Income Net Of Tax	(10)	(37)	(38)
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease	(73)	164	105
Other Comprehensive Income (Loss) Net Of Tax	(73)	164	105
Total Accumulated Other Comprehensive Income - Ending Balance	412	485	321
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains on cash flow hedge derivatives arising during period	37	89	2
Other Comprehensive Income Loss Reclassification Adjustment On Derivatives Included In Net Income Before Tax	(83)	(69)	(60)
Other Comprehensive Income Loss Derivatives Qualifying As Hedges Before Tax	(46)	20	(58)
Unrealized holding gains on cash flow hedge derivatives arising during period	(13)	(31)	(1)
Other Comprehensive Income Loss Reclassification Adjustment On Derivatives Included In Net Income Tax	(29)	(24)	(21)
Other Comprehensive Income Loss Derivatives Qualifying As Hedges Tax	16	(7)	20
Total Accumulated Other Comprehensive Income - Beginning Balance	80	67	105
Unrealized holding gains (losses) on cash flow hedge derivatives arising during period	24	58	1
Other Comprehensive Income Loss Reclassification Adjustment On Derivatives Included In Net Income Net Of Tax	(54)	(45)	(39)
Other Comprehensive Income Loss Derivatives Qualifying As Hedges Net Of Tax	(30)	13	(38)
Other Comprehensive Income (Loss) Net Of Tax	(30)	13	(38)
Total Accumulated Other Comprehensive Income - Ending Balance	50	80	67
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net prior service cost			1
Net actuarial loss (gain)	(13)	32	(10)
Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Adjustment Before Tax	13	(32)	11
Net prior service cost			(1)
Net actuarial loss	5	11	(4)
Other Comprehensive Income Loss Pension And Other Postretirement Benefit Plans Tax	(5)	(11)	5
Total Accumulated Other Comprehensive Income - Beginning Balance	(95)	(74)	(80)
Net actuarial loss (gain)	(8)	21	(6)
Other Comprehensive Income Loss Pension And Other Postretiremen tBenefit Plans Adjustment Net Of Tax	8	(21)	6
Other Comprehensive Income (Loss) Net Of Tax	(8)	21	(6)
Total Accumulated Other Comprehensive Income - Ending Balance	$ (87)	$ (95)	$ (74)