Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Accumulated other comprehensive income [Roll Forward]    
Balance, beginning of year $ 25,943  
Other comprehensive income (loss), net of taxes (689) $ 1,177
Balance, end of period 25,204 24,340
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes (826) 1,483
Income tax expense (benefit) (137) 306
Other comprehensive income (loss), net of taxes (689) 1,177
Accumulated Other Comprehensive Income (Loss) [Member]    
Accumulated other comprehensive income [Roll Forward]    
Balance, beginning of year 640 (1,859)
Other comprehensive income (loss) before reclassifications (708)  
Amounts reclassified from accumulated other comprehensive income 19  
Other comprehensive income (loss), net of taxes (689) 1,177
Balance, end of period (49) (682)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes (826) 1,483
Income tax expense (benefit) (137) 306
Other comprehensive income (loss), net of taxes (689) 1,177
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 2,057  
Other comprehensive income (loss) before reclassifications (459)  
Amounts reclassified from accumulated other comprehensive income 2  
Other comprehensive income (loss), net of taxes (457) 1,116
Balance, end of period 1,600  
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes (575) 1,416
Income tax expense (benefit) (118) 300
Other comprehensive income (loss), net of taxes (457) 1,116
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 189  
Other comprehensive income (loss) before reclassifications (4)  
Amounts reclassified from accumulated other comprehensive income 0  
Other comprehensive income (loss), net of taxes (4) 4
Balance, end of period 185  
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes (6) 5
Income tax expense (benefit) (2) 1
Other comprehensive income (loss), net of taxes (4) 4
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]    
Accumulated other comprehensive income [Roll Forward]    
Balance, beginning of year (846)  
Other comprehensive income (loss) before reclassifications 1  
Amounts reclassified from accumulated other comprehensive income 17  
Other comprehensive income (loss), net of taxes 18 10
Balance, end of period (828)  
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes 23 12
Income tax expense (benefit) 5 2
Other comprehensive income (loss), net of taxes 18 10
Net Unrealized Foreign Currency Translation [Member]    
Accumulated other comprehensive income [Roll Forward]    
Balance, beginning of year (760)  
Other comprehensive income (loss) before reclassifications (246)  
Amounts reclassified from accumulated other comprehensive income 0  
Other comprehensive income (loss), net of taxes (246) 47
Balance, end of period (1,006)  
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)    
Other comprehensive income (loss) before income taxes (268) 50
Income tax expense (benefit) (22) 3
Other comprehensive income (loss), net of taxes $ (246) $ 47