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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on sale of investment securities	$ 25	$ 10	$ 47	$ 25
Amortization of unrealized gains on securities transferred from available-for-sale to held-to-maturity, Interest income	4,437	4,123	13,239	11,862
Realized gains (losses) on derivative hedges	(1,156)	(872)	(3,394)	(2,246)
Actuarial gains (losses) and prior service cost (credit) amortization	(330)	(377)	(1,077)	(1,194)
Applicable income taxes	(467)	(460)	(1,294)	(1,263)
Net income	1,917	1,822	5,453	5,262
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(1)	(15)	(4)	(52)
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	10	47	25
Applicable income taxes	(6)	(2)	(12)	(6)
Net income	19	8	35	19
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Investment Securities Transferred From Available For Sale to Held To Maturity [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized gains on securities transferred from available-for-sale to held-to-maturity, Interest income	1	2	5	7
Applicable income taxes	0		(1)	(1)
Net income	1	2	4	6
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	(6)	2	8	1
Applicable income taxes	1	(1)	(2)	(1)
Net income	(5)	1	6
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	(22)	(34)	(66)	(103)
Applicable income taxes	6	8	17	26
Net income	$ (16)	$ (26)	$ (49)	$ (77)