Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	$ 23,731			$ 23,731
Other comprehensive income (loss), net of taxes		$ (315)	$ 59	(1,614)	$ 470
Balance, end of period		22,460	23,738	22,460	23,738
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		(403)	64	(2,027)	660
Income tax expense (benefit)		(88)	5	(413)	190
Other comprehensive income (loss), net of taxes		(315)	59	(1,614)	470
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(343)			(343)	(755)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, pre-tax	(34)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, tax	(12)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, net of income taxes	(22)					$ (22)
Reclassification of certain tax effects from adoption of updated accounting guidance	(24)			(24)
Effect of adoption of updated accounting guidance, net of tax, total	(46)
Other comprehensive income (loss) before reclassifications				(1,641)
Amounts reclassified from accumulated other comprehensive income				27
Other comprehensive income (loss), net of taxes		(315)	59	(1,614)	470
Balance, end of period		(2,003)	(285)	(2,003)	(285)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		(403)	64	(2,027)	660
Income tax expense (benefit)		(88)	5	(413)	190
Other comprehensive income (loss), net of taxes		(315)	59	(1,614)	470
Changes in Net Unrealized Gains (Losses) on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	747			747
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, pre-tax	(34)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, tax	(12)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, net of income taxes	(22)
Reclassification of certain tax effects from adoption of updated accounting guidance	145
Effect of adoption of updated accounting guidance, net of tax, total	123
Other comprehensive income (loss) before reclassifications				(1,488)
Amounts reclassified from accumulated other comprehensive income				(24)
Other comprehensive income (loss), net of taxes		(327)	(30)	(1,512)	274
Balance, end of period		(642)		(642)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		(414)	(42)	(1,915)	429
Income tax expense (benefit)		(87)	(12)	(403)	155
Other comprehensive income (loss), net of taxes		(327)	(30)	(1,512)	274
Changes in Net Unrealized Gains (Losses) on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	207			207
Reclassification of certain tax effects from adoption of updated accounting guidance	7
Effect of adoption of updated accounting guidance, net of tax, total	7
Other comprehensive income (loss) before reclassifications				(19)
Other comprehensive income (loss), net of taxes		(8)	1	(19)	2
Balance, end of period		195		195
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		(11)	2	(25)	4
Income tax expense (benefit)		(3)	1	(6)	2
Other comprehensive income (loss), net of taxes		(8)	1	(19)	2
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(686)			(686)
Reclassification of certain tax effects from adoption of updated accounting guidance	(141)
Effect of adoption of updated accounting guidance, net of tax, total	(141)
Amounts reclassified from accumulated other comprehensive income				51
Other comprehensive income (loss), net of taxes		17	(6)	51	17
Balance, end of period		(776)		(776)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		22	(9)	65	25
Income tax expense (benefit)		5	(3)	14	8
Other comprehensive income (loss), net of taxes		17	(6)	51	17
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(611)			(611)
Reclassification of certain tax effects from adoption of updated accounting guidance	(35)
Effect of adoption of updated accounting guidance, net of tax, total	$ (35)
Other comprehensive income (loss) before reclassifications				(134)
Other comprehensive income (loss), net of taxes		3	94	(134)	177
Balance, end of period		(780)		(780)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes			113	(152)	202
Income tax expense (benefit)		(3)	19	(18)	25
Other comprehensive income (loss), net of taxes		$ 3	$ 94	$ (134)	$ 177