Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax		$ 414	$ 384	$ 135	$ 298
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax		237	141	87	97
Accumulated Other Comprehensive Income (Loss), Financial Instruments, net of tax		(7)	0	0	0
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax		(623)	(590)	(682)	(517)
Accumulated Other Comprehensive Income (Loss), Net of Tax		21	(65)	$ (460)	$ (122)
Cumulative effect of credit risk adjustment		0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		279	86
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		192	78
Credit Risk Adjustment		(2)	0
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		469	164
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		(42)	(34)
Other Comprehensive Income Loss Reclassfication Adjustment From AOCI on Long Term Debt, Net of Tax		0	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax		59	(73)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		17	(107)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax		279	86
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax		150	44
Other Comprehensive Income Loss Long Term Debt, Net of Tax		(2)	0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax		59	(73)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		486	$ 57
AOCI Attributable to Parent [Member]
Cumulative effect of credit risk adjustment	[1]	$ (5)

[1] Related to the Company's early adoption of the ASU 2016-01 provision related to changes in instrument-specific credit risk, for the three months ended March 31, 2016. See Note 1, "Significant Accounting Policies," and Note 17, "Accumulated Other Comprehensive Income/(Loss)," for additional information.