Accumulated Other Comprehensive Income (Loss) - Impact to Net Income for Items Reclassified out of Accumulated Other Comprehensive Income and into Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on sale of investment securities	$ 43	$ 81	$ 68	$ 131
Realized gains (losses) on derivative hedges	(245)	(472)	(523)	(1,365)
Actuarial gains (losses) and prior service cost (credit) amortization	(329)	(451)	(646)	(943)
Applicable income taxes	(551)	(64)	(1,158)	(324)
Net income	1,988	695	4,273	1,874
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	8	44	(5)	48
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on sale of investment securities	43	81	68	131
Applicable income taxes	(11)	(20)	(17)	(33)
Net income	32	61	51	98
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Derivative Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on derivative hedges	8	9	4	(4)
Applicable income taxes	(2)	(2)	(1)	1
Net income	6	7	3	(3)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Retirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Actuarial gains (losses) and prior service cost (credit) amortization	(40)	(31)	(79)	(62)
Applicable income taxes	10	7	20	15
Net income	$ (30)	$ (24)	$ (59)	$ (47)