Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of period	$ 29,201
Other comprehensive loss, net of taxes	(1,211)	$ (689)
Balance, end of period	28,269	25,204
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	(1,545)	(826)
Income tax expense (benefit)	(334)	(137)
Other comprehensive loss, net of taxes	(1,211)	(689)
Accumulated Other Comprehensive Income [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of period	2,502	640
Other comprehensive income (loss) before reclassifications	(1,220)
Amounts reclassified from accumulated other comprehensive income, net of tax	9
Other comprehensive loss, net of taxes	(1,211)	(689)
Balance, end of period	1,291	(49)
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	(1,545)	(826)
Income tax expense (benefit)	(334)	(137)
Other comprehensive loss, net of taxes	(1,211)	(689)
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 period	3,892
Other comprehensive income (loss) before reclassifications	(1,246)
Amounts reclassified from accumulated other comprehensive income, net of tax	(11)
Other comprehensive loss, net of taxes	(1,257)	(457)
Balance, end of period	2,635
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	(1,596)	(575)
Income tax expense (benefit)	(339)	(118)
Other comprehensive loss, net of taxes	(1,257)	(457)
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 period	182
Other comprehensive income (loss) before reclassifications	0
Amounts reclassified from accumulated other comprehensive income, net of tax	0
Other comprehensive loss, net of taxes	0	(4)
Balance, end of period	182
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	0	(6)
Income tax expense (benefit)	0	(2)
Other comprehensive loss, net of taxes	0	(4)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of period	(832)
Other comprehensive income (loss) before reclassifications	0
Amounts reclassified from accumulated other comprehensive income, net of tax	20
Other comprehensive loss, net of taxes	20	18
Balance, end of period	(812)
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	25	23
Income tax expense (benefit)	5	5
Other comprehensive loss, net of taxes	20	18
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of period	(740)
Other comprehensive income (loss) before reclassifications	26
Amounts reclassified from accumulated other comprehensive income, net of tax	0
Other comprehensive loss, net of taxes	26	(246)
Balance, end of period	(714)
Pre-tax components of other comprehensive loss and the related income tax expense (benefit)
Other comprehensive income (loss), before taxes	26	(268)
Income tax expense (benefit)	0	(22)
Other comprehensive loss, net of taxes	$ 26	$ (246)