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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ 503	$ (1,773)	$ (1,373)	$ (2,322)
Changes in unrealized gains and losses	453	721	3,240	1,506
Changes in unrealized gains and losses		(135)	(257)	(209)
Changes in unrealized gains and losses	453	586	2,983	1,297
Foreign currency translation adjustment	1	(8)	(12)	8
Reclassification to earnings of realized gains and losses	(59)	(4)	(65)	4
Applicable income taxes	(100)	(145)	(735)	(331)
Balance at end of period	798	(1,344)	798	(1,344)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(75)	(72)	(65)	(84)
Foreign currency translation adjustment	1	(8)	(12)	8
Applicable income taxes		2	3	(2)
Balance at end of period	(74)	(78)	(74)	(78)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,613)	(1,402)	(1,636)	(1,418)
Reclassification to earnings of realized gains and losses	31	22	62	44
Applicable income taxes	(7)	(5)	(15)	(11)
Balance at end of period	(1,589)	(1,385)	(1,589)	(1,385)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(233)	51	(51)	112
Changes in unrealized gains and losses		(135)	(257)	(209)
Reclassification to earnings of realized gains and losses	(9)	(6)	4	(14)
Applicable income taxes	2	35	64	56
Balance at end of period	(240)	(55)	(240)	(55)
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		13		14
Reclassification to earnings of realized gains and losses		(3)		(4)
Applicable income taxes		1		1
Balance at end of period		11		11
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	2,424	(363)	379	(946)
Changes in unrealized gains and losses	453	721	3,240	1,506
Reclassification to earnings of realized gains and losses	(81)	(17)	(131)	(22)
Applicable income taxes	(95)	(178)	(787)	(375)
Balance at end of period	$ 2,701	$ 163	$ 2,701	$ 163