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, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (1,373)	$ (2,322)
Changes in unrealized gains and losses	2,787	785
Changes in unrealized gains and losses	(257)	(74)
Changes in unrealized gains and losses	2,530	711
Foreign currency translation adjustment	(13)	16
Reclassification to earnings of realized gains and losses	(6)	8
Applicable income taxes	(635)	(186)
Balance at end of period	503	(1,773)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(65)	(84)
Foreign currency translation adjustment	(13)	16
Applicable income taxes	3	(4)
Balance at end of period	(75)	(72)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,636)	(1,418)
Reclassification to earnings of realized gains and losses	31	22
Applicable income taxes	(8)	(6)
Balance at end of period	(1,613)	(1,402)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(51)	112
Changes in unrealized gains and losses	(257)	(74)
Reclassification to earnings of realized gains and losses	13	(8)
Applicable income taxes	62	21
Balance at end of period	(233)	51
Unrealized Gains (Losses) on Investment Securities Transferred From Available For Sale to Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period		14
Reclassification to earnings of realized gains and losses		(1)
Balance at end of period		13
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	379	(946)
Changes in unrealized gains and losses	2,787	785
Reclassification to earnings of realized gains and losses	(50)	(5)
Applicable income taxes	(692)	(197)
Balance at end of period	$ 2,424	$ (363)