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Loans and Related Allowance for Credit Losses (Narrative) (Details)
$ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
portfolio_segment
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Number of portfolio segments | portfolio_segment 2      
Allowance for credit loss $ 5,221 $ 4,394 $ 5,945 $ 3,060
Increase (decrease) in allowance for credit loss (700) 1,300    
Performing Financial Instruments        
Troubled debt restructurings (TDRs) 700   700  
TDRs        
Amortized Cost - Subsequently Defaulted TDRs 15 $ 29    
TDRs | Nonperforming loans        
Troubled debt restructurings (TDRs) 900   900  
Federal Reserve Bank        
Loans pledged as collateral for the ability to borrow 28,600   30,100  
Federal Home Loan Bank        
Loans pledged as collateral for the ability to borrow $ 66,900   $ 69,000