Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (65)	$ (200)	$ (122)	$ (289)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	86	107
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	78	14
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0	0
Other Comprehensive Income (Loss), Total Unrealized Gain (Loss) Arising During Period, Net of Tax	164	121
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	1
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(34)	(64)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	(73)	31
Other Comprehensive Income (Loss), Total Reclassification Adjustments from AOCI, Net of Tax	(107)	(32)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	86	108
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	44	(50)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	(73)	31
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	57	89
Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	384	31	298	(77)
Derivative [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	141	229	97	279
Pension and Other Postretirement Plans Costs [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (590)	$ (460)	$ (517)	$ (491)