Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year		$ 25,943	$ 22,894
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201601Member
Shareholders' equity		$ 29,201	22,894	$ 22,894
Other comprehensive income (loss), net of taxes		1,862	2,499	(1,470)
Balance, end of year		29,201	25,943	22,894
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		2,352	3,140	(1,819)
Income tax expense (benefit)		490	641	(349)
Other comprehensive income (loss), net of taxes		1,862	2,499	(1,470)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	$ (343)	640	(1,859)	(343)
Shareholders' equity	(343)	2,502	640	(343)	$ (343)
Reclassification of certain tax effects from adoption of updated accounting guidance	(24)
Other comprehensive income (loss) before reclassifications		1,837	2,494	(1,513)
Amounts reclassified from accumulated other comprehensive income		25	5	43
Other comprehensive income (loss), net of taxes		1,862	2,499	(1,470)
Balance, end of year		2,502	640	(1,859)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		2,352	3,140	(1,819)
Income tax expense (benefit)		490	641	(349)
Other comprehensive income (loss), net of taxes		1,862	2,499	(1,470)
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment, Before Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(34)			(34)
Shareholders' equity	(34)			(34)	(34)
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment, Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(12)			(12)
Shareholders' equity	(12)			(12)	(12)
Accumulated Other Comprehensive Income (Loss) [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(22)			(22)
Shareholders' equity	(22)			(22)	(22)
Effect of adoption of updated accounting guidance, net of tax, total					(46)
Balance, end of year	(22)
Changes in Net Unrealized Gains 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	2,057	(306)	747
Shareholders' equity	747	3,892	2,057	(306)	747
Reclassification of certain tax effects from adoption of updated accounting guidance	145
Other comprehensive income (loss) before reclassifications		1,876	2,406	(1,151)
Amounts reclassified from accumulated other comprehensive income		(41)	(43)	(25)
Other comprehensive income (loss), net of taxes		1,835	2,363	(1,176)
Balance, end of year		3,892	2,057	(306)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		2,331	2,994	(1,489)
Income tax expense (benefit)		496	631	(313)
Other comprehensive income (loss), net of taxes		1,835	2,363	(1,176)
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment, Before Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(34)			(34)
Shareholders' equity	(34)			(34)	(34)
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment, Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(12)			(12)
Shareholders' equity	(12)			(12)	(12)
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(22)			(22)
Shareholders' equity	(22)			(22)	(22)
Effect of adoption of updated accounting guidance, net of tax, total					123
Changes in Net Unrealized Gains 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	189	193	207
Shareholders' equity	207	182	189	193	207
Reclassification of certain tax effects from adoption of updated accounting guidance	7
Other comprehensive income (loss) before reclassifications		(7)	(4)	(21)
Amounts reclassified from accumulated other comprehensive income		0	0	0
Other comprehensive income (loss), net of taxes		(7)	(4)	(21)
Balance, end of year		182	189	193
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		(9)	(4)	(27)
Income tax expense (benefit)		(2)	0	(6)
Other comprehensive income (loss), net of taxes		(7)	(4)	(21)
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment, Before Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment, Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Effect of adoption of updated accounting guidance, net of tax, total					7
Net Benefit Plan Assets and Obligations Recognized in Shareholders’ Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(686)	(846)	(873)	(686)
Shareholders' equity	(686)	(832)	(846)	(873)	(686)
Reclassification of certain tax effects from adoption of updated accounting guidance	(141)
Other comprehensive income (loss) before reclassifications		(53)	(14)	(114)
Amounts reclassified from accumulated other comprehensive income		67	41	68
Other comprehensive income (loss), net of taxes		14	27	(46)
Balance, end of year		(832)	(846)	(873)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		18	33	(56)
Income tax expense (benefit)		4	6	(10)
Other comprehensive income (loss), net of taxes		14	27	(46)
Net Benefit Plan Assets and Obligations Recognized in Shareholders’ Equity [Member] | Cumulative Effect, Period of Adoption, Adjustment, Before Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Net Benefit Plan Assets and Obligations Recognized in Shareholders’ Equity [Member] | Cumulative Effect, Period of Adoption, Adjustment, Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Net Benefit Plan Assets and Obligations Recognized in Shareholders’ Equity [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Effect of adoption of updated accounting guidance, net of tax, total					(141)
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(611)	(760)	(873)	(611)
Shareholders' equity	(611)	(740)	(760)	(873)	(611)
Reclassification of certain tax effects from adoption of updated accounting guidance	(35)
Other comprehensive income (loss) before reclassifications		21	106	(227)
Amounts reclassified from accumulated other comprehensive income		(1)	7	0
Other comprehensive income (loss), net of taxes		20	113	(227)
Balance, end of year		(740)	(760)	(873)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes		12	117	(247)
Income tax expense (benefit)		(8)	4	(20)
Other comprehensive income (loss), net of taxes		$ 20	$ 113	(227)
Net Unrealized Foreign Currency Translation [Member] | Cumulative Effect, Period of Adoption, Adjustment, Before Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Net Unrealized Foreign Currency Translation [Member] | Cumulative Effect, Period of Adoption, Adjustment, Tax [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	0			0	0
Net Unrealized Foreign Currency Translation [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	0			0
Shareholders' equity	$ 0			$ 0	0
Effect of adoption of updated accounting guidance, net of tax, total					$ (35)