Accumulated Other Comprehensive Income - Changes in AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 15,240
Total other comprehensive income (loss), net of tax	$ 0	$ (68)	35	$ 223
Ending Balance	15,250		15,250
Unrealized Gains (Losses) on Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(145)	129	(185)	(58)
Other comprehensive income before reclassification, net of income taxes	0	(28)	40	159
Amounts reclassified from accumulated other comprehensive income, net of income taxes	0	0	0	0
Total other comprehensive income (loss), net of tax	0	(28)	40	159
Ending Balance	(145)	101	(145)	101
Unrealized Gains (Losses) on Derivative Financial Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(31)	114	(14)	20
Other comprehensive income before reclassification, net of income taxes	(10)	(28)	(16)	93
Amounts reclassified from accumulated other comprehensive income, net of income taxes	0	(13)	(11)	(40)
Total other comprehensive income (loss), net of tax	(10)	(41)	(27)	53
Ending Balance	(41)	73	(41)	73
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	4	5	(3)	(2)
Other comprehensive income before reclassification, net of income taxes	7	(2)	14	5
Amounts reclassified from accumulated other comprehensive income, net of income taxes	0	0	0	0
Total other comprehensive income (loss), net of tax	7	(2)	14	5
Ending Balance	11	3	11	3
Net Pension and Postretirement Benefit Costs [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(334)	(362)	(339)	(365)
Other comprehensive income before reclassification, net of income taxes	0	1	0	(1)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	3	2	8	7
Total other comprehensive income (loss), net of tax	3	3	8	6
Ending Balance	(331)	(359)	(331)	(359)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(506)	(114)	(541)	(405)
Other comprehensive income before reclassification, net of income taxes	(3)	(57)	38	256
Amounts reclassified from accumulated other comprehensive income, net of income taxes	3	(11)	(3)	(33)
Total other comprehensive income (loss), net of tax	0	(68)	35	223
Ending Balance	$ (506)	$ (182)	$ (506)	$ (182)