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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (2,193)	$ (1,439)	$ (1,404)	$ (1,535)
Revaluation of tax related balances			(300)
Changes in unrealized gains and losses	(212)	328	(988)	455
Changes in unrealized gains and losses	33	(37)	119	(30)
Changes in unrealized gains and losses	2		(1)
Changes in unrealized gains and losses	(177)	291	(870)	425
Foreign currency translation adjustment	(8)	(1)	5	9
Reclassification to earnings of realized gains and losses	21	26	50	37
Applicable income taxes	42	(123)	204	(182)
Balance at end of period	(2,315)	(1,246)	(2,315)	(1,246)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(76)	(64)	(69)	(71)
Revaluation of tax related balances			(13)
Foreign currency translation adjustment	(8)	(1)	5	9
Applicable income taxes	3	(2)	(4)	(5)
Balance at end of period	(81)	(67)	(81)	(67)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,271)	(1,095)	(1,066)	(1,113)
Revaluation of tax related balances			(229)
Changes in unrealized gains and losses	2		(1)
Reclassification to earnings of realized gains and losses	35	29	69	58
Applicable income taxes	(10)	(11)	(17)	(22)
Balance at end of period	(1,244)	(1,077)	(1,244)	(1,077)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	152	68	71	55
Revaluation of tax related balances			15
Changes in unrealized gains and losses	33	(37)	119	(30)
Reclassification to earnings of realized gains and losses	(2)	10	1	24
Applicable income taxes	(7)	10	(30)	2
Balance at end of period	176	51	176	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	19	23	17	25
Revaluation of tax related balances			4
Reclassification to earnings of realized gains and losses	(2)	(4)	(5)	(7)
Applicable income taxes		2	1	3
Balance at end of period	17	21	17	21
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,017)	(371)	(357)	(431)
Revaluation of tax related balances			(77)
Changes in unrealized gains and losses	(212)	328	(988)	455
Reclassification to earnings of realized gains and losses	(10)	(9)	(15)	(38)
Applicable income taxes	56	(122)	254	(160)
Balance at end of period	$ (1,183)	$ (174)	$ (1,183)	$ (174)