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, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (1,404)	$ (1,535)
Revaluation of tax related balances	(300)
Changes in unrealized gains and losses	(776)	127
Changes in unrealized gains and losses	86	7
Changes in unrealized gains and losses	(3)
Changes in unrealized gains and losses	(693)	134
Foreign currency translation adjustment	13	10
Reclassification to earnings of realized gains and losses	29	11
Applicable income taxes	162	(59)
Balance at end of period	(2,193)	(1,439)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(69)	(71)
Revaluation of tax related balances	(13)
Foreign currency translation adjustment	13	10
Applicable income taxes	(7)	(3)
Balance at end of period	(76)	(64)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,066)	(1,113)
Revaluation of tax related balances	(229)
Changes in unrealized gains and losses	(3)
Reclassification to earnings of realized gains and losses	34	29
Applicable income taxes	(7)	(11)
Balance at end of period	(1,271)	(1,095)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	71	55
Revaluation of tax related balances	15
Changes in unrealized gains and losses	86	7
Reclassification to earnings of realized gains and losses	3	14
Applicable income taxes	(23)	(8)
Balance at end of period	152	68
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	17	25
Revaluation of tax related balances	4
Reclassification to earnings of realized gains and losses	(3)	(3)
Applicable income taxes	1	1
Balance at end of period	19	23
Unrealized Gains (Losses) on Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(357)	(431)
Revaluation of tax related balances	(77)
Changes in unrealized gains and losses	(776)	127
Reclassification to earnings of realized gains and losses	(5)	(29)
Applicable income taxes	198	(38)
Balance at end of period	$ (1,017)	$ (371)