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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains (losses) on sale of investment securities	$ 81	$ 17	$ 131	$ 22
Amortization of unrealized gains on securities transferred from available-for-sale to held-to-maturity, Interest income	3,672	4,451	7,788	8,802
Realized gains (losses) on derivative hedges	(472)	(1,146)	(1,365)	(2,238)
Actuarial gains (losses) and prior service cost (credit) amortization	(451)	(382)	(943)	(747)
Applicable income taxes	(64)	(449)	(324)	(827)
Net income	695	1,828	1,874	3,536
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	44	3	48	(3)
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	81	17	131	22
Applicable income taxes	(20)	(4)	(33)	(6)
Net income	61	13	98	16
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		3		4
Applicable income taxes		(1)		(1)
Net income		2		3
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	9	6	(4)	14
Applicable income taxes	(2)	(1)	1	(3)
Net income	7	5	(3)	11
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	(31)	(22)	(62)	(44)
Applicable income taxes	7	5	15	11
Net income	$ (24)	$ (17)	$ (47)	$ (33)