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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (1,246)	$ (562)	$ (1,535)	$ (1,019)
Changes in unrealized gains and losses	24	(105)	479	716
Changes in unrealized gains and losses	(3)	31	(33)	(152)
Changes in unrealized gains and losses	21	(74)	446	564
Other-than-temporary impairment not recognized in earnings on securities available-for-sale				(1)
Foreign currency translation adjustment	2	6	11	(30)
Reclassification to earnings of realized gains and losses	21	54	58	196
Applicable income taxes	(17)	(3)	(199)	(289)
Balance at end of period	(1,219)	(579)	(1,219)	(579)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(67)	(66)	(71)	(43)
Foreign currency translation adjustment	2	6	11	(30)
Applicable income taxes		(10)	(5)	3
Balance at end of period	(65)	(70)	(65)	(70)
Unrealized Gains (Losses) on Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,077)	(1,006)	(1,113)	(1,056)
Changes in unrealized gains and losses	0	0	0	0
Reclassification to earnings of realized gains and losses	29	41	87	122
Applicable income taxes	(11)	(16)	(33)	(47)
Balance at end of period	(1,059)	(981)	(1,059)	(981)
Unrealized Gains (Losses) on Derivative Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	51	(133)	55	(67)
Changes in unrealized gains and losses	(3)	31	(33)	(152)
Reclassification to earnings of realized gains and losses	4	28	28	104
Applicable income taxes		(22)	2	19
Balance at end of period	52	(96)	52	(96)
Unrealized Gains (Losses) on Securities Transferred From Available For Sale to Held To Maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	21	31	25	36
Reclassification to earnings of realized gains and losses	(3)	(5)	(10)	(14)
Applicable income taxes	1	1	4	5
Balance at end of period	19	27	19	27
Unrealized Gains (Losses) on Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(174)	612	(431)	111
Changes in unrealized gains and losses	24	(105)	479	716
Other-than-temporary impairment not recognized in earnings on securities available-for-sale				(1)
Reclassification to earnings of realized gains and losses	(9)	(10)	(47)	(16)
Applicable income taxes	(7)	44	(167)	(269)
Balance at end of period	$ (166)	$ 541	$ (166)	$ 541