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
	Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on sale of investment securities	$ 25	$ 50
Realized gains (losses) on derivative hedges	(278)	(893)
Actuarial gains (losses) and prior service cost (credit) amortization	(317)	(492)
Applicable income taxes	(607)	(260)
Net income	2,285	1,179
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(13)	4
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	25	50
Applicable income taxes	(6)	(13)
Net income	19	37
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	(4)	(13)
Applicable income taxes	1	3
Net income	(3)	(10)
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	(39)	(31)
Applicable income taxes	10	8
Net income	$ (29)	$ (23)