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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 798	$ (1,344)	$ (1,373)	$ (2,322)
Changes in unrealized gains and losses	(305)	284	2,935	1,790
Changes in unrealized gains and losses	27	(59)	(230)	(268)
Changes in unrealized gains and losses	(278)	225	2,705	1,522
Foreign currency translation adjustment	6	2	(6)	10
Reclassification to earnings of realized gains and losses	23	2	(42)	6
Applicable income taxes	63	(58)	(672)	(389)
Balance at end of period	612	(1,173)	612	(1,173)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(74)	(78)	(65)	(84)
Foreign currency translation adjustment	6	2	(6)	10
Applicable income taxes	(2)		1	(2)
Balance at end of period	(70)	(76)	(70)	(76)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,589)	(1,385)	(1,636)	(1,418)
Reclassification to earnings of realized gains and losses	32	22	94	66
Applicable income taxes	(9)	(6)	(24)	(17)
Balance at end of period	(1,566)	(1,369)	(1,566)	(1,369)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(240)	(55)	(51)	112
Changes in unrealized gains and losses	27	(59)	(230)	(268)
Reclassification to earnings of realized gains and losses	3	6	7	(8)
Applicable income taxes	(7)	14	57	70
Balance at end of period	(217)	(94)	(217)	(94)
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		11		14
Reclassification to earnings of realized gains and losses		(1)		(5)
Applicable income taxes				1
Balance at end of period		10		10
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	2,701	163	379	(946)
Changes in unrealized gains and losses	(305)	284	2,935	1,790
Reclassification to earnings of realized gains and losses	(12)	(25)	(143)	(47)
Applicable income taxes	81	(66)	(706)	(441)
Balance at end of period	$ 2,465	$ 356	$ 2,465	$ 356