Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	$ 302	$ 169	$ 302	$ 169	$ 183	$ 298	$ 206	$ (77)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	191	111	191	111	107	97	193	279
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(581)	(457)	(581)	(457)	(584)	(517)	(458)	(491)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(88)	(177)	(88)	(177)	$ (294)	$ (122)	$ (59)	$ (289)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	123	(43)	17	239
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	128	(19)	212	23
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Total Unrealized Gain (Loss) Arising During Period, Net of Tax	251	(62)	229	262
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(4)	6	(13)	7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(44)	(63)	(118)	(191)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	3	1	(64)	34
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(45)	(56)	(195)	(150)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	119	(37)	4	246
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	84	(82)	94	(168)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	3	1	(64)	34
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 206	$ (118)	$ 34	$ 112