Accumulated Other Comprehensive Income (Loss) - Reconciliation of Transactions Affecting Accumulated Other Comprehensive Income (Loss) Included in Shareholders' Equity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 322	$ (1,373)
Changes in unrealized gains and losses	(3,378)	2,787
Changes in unrealized gains and losses	99	(257)
Changes in unrealized gains and losses	(3,279)	2,530
Foreign currency translation adjustment	25	(13)
Reclassification to earnings of realized gains and losses	18	(6)
Applicable income taxes	819	(635)
Balance at end of period	(2,095)	503
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	2,417	379
Changes in unrealized gains and losses	(3,378)	2,787
Reclassification to earnings of realized gains and losses	(25)	(50)
Applicable income taxes	861	(692)
Balance at end of period	(125)	2,424
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(189)	(51)
Changes in unrealized gains and losses	99	(257)
Reclassification to earnings of realized gains and losses	4	13
Applicable income taxes	(26)	62
Balance at end of period	(112)	(233)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,842)	(1,636)
Changes in unrealized gains and losses	0
Reclassification to earnings of realized gains and losses	39	31
Applicable income taxes	(10)	(8)
Balance at end of period	(1,813)	(1,613)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(64)	(65)
Foreign currency translation adjustment	25	(13)
Applicable income taxes	(6)	3
Balance at end of period	$ (45)	$ (75)