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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (1,202)	$ 798	$ 322	$ (1,373)
Changes in unrealized gains and losses	(825)	(305)	(3,008)	2,935
Changes in unrealized gains and losses	8	27	121	(230)
Changes in unrealized gains and losses	(817)	(278)	(2,887)	2,705
Foreign currency translation adjustment	(1)	6	23	(6)
Reclassification to earnings of realized gains and losses	27	23	34	(42)
Applicable income taxes	201	63	716	(672)
Balance at end of period	(1,792)	612	(1,792)	612
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	735	2,701	2,417	379
Changes in unrealized gains and losses	(825)	(305)	(3,008)	2,935
Reclassification to earnings of realized gains and losses	(20)	(12)	(88)	(143)
Applicable income taxes	215	81	784	(706)
Balance at end of period	105	2,465	105	2,465
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(107)	(240)	(189)	(51)
Changes in unrealized gains and losses	8	27	121	(230)
Reclassification to earnings of realized gains and losses	8	3	4	7
Applicable income taxes	(4)	(7)	(31)	57
Balance at end of period	(95)	(217)	(95)	(217)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,783)	(1,589)	(1,842)	(1,636)
Reclassification to earnings of realized gains and losses	39	32	118	94
Applicable income taxes	(10)	(9)	(30)	(24)
Balance at end of period	(1,754)	(1,566)	(1,754)	(1,566)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(47)	(74)	(64)	(65)
Foreign currency translation adjustment	(1)	6	23	(6)
Applicable income taxes		(2)	(7)	1
Balance at end of period	$ (48)	$ (70)	$ (48)	$ (70)