Accumulated Other Comprehensive Income - Changes in AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 15,025
Other comprehensive income before reclassification, net of income taxes	$ (97)	$ (3)	(424)	$ 38
Amounts reclassified from accumulated other comprehensive income, net of income taxes	16	3	42	(3)
Other amounts reclassified from AOCI, net of income taxes	0		(5)
Total other comprehensive income (loss), net of tax	(81)	0	(387)	35
Ending Balance	15,210		15,210
Unrealized Gains (Losses) on Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(527)	(145)	(311)	(185)
Other comprehensive income before reclassification, net of income taxes	(72)	0	(288)	40
Amounts reclassified from accumulated other comprehensive income, net of income taxes	0	0	0	0
Other amounts reclassified from AOCI, net of income taxes	0		0
Total other comprehensive income (loss), net of tax	(72)	0	(288)	40
Ending Balance	(599)	(145)	(599)	(145)
Unrealized Gains (Losses) on Derivative Financial Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(166)	(31)	(86)	(14)
Other comprehensive income before reclassification, net of income taxes	(26)	(10)	(126)	(16)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	14	0	34	(11)
Other amounts reclassified from AOCI, net of income taxes	0		0
Total other comprehensive income (loss), net of tax	(12)	(10)	(92)	(27)
Ending Balance	(178)	(41)	(178)	(41)
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(7)	4	9	(3)
Other comprehensive income before reclassification, net of income taxes	1	7	(10)	14
Amounts reclassified from accumulated other comprehensive income, net of income taxes	0	0	0	0
Other amounts reclassified from AOCI, net of income taxes	0		(5)
Total other comprehensive income (loss), net of tax	1	7	(15)	14
Ending Balance	(6)	11	(6)	11
Net Pension and Postretirement Benefit Costs [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(385)	(334)	(391)	(339)
Other comprehensive income before reclassification, net of income taxes	0	0	0	0
Amounts reclassified from accumulated other comprehensive income, net of income taxes	2	3	8	8
Other amounts reclassified from AOCI, net of income taxes	0		0
Total other comprehensive income (loss), net of tax	2	3	8	8
Ending Balance	(383)	(331)	(383)	(331)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,085)	(506)	(779)	(541)
Total other comprehensive income (loss), net of tax	(81)	0	(387)	35
Ending Balance	$ (1,166)	$ (506)	$ (1,166)	$ (506)