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Loans and Related Allowance for Credit Losses (Narrative) (Details)
$ in Millions
9 Months Ended
Sep. 30, 2021
USD ($)
portfolio_segment
Dec. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
Number of portfolio segments | portfolio_segment 2    
Financing receivable, threshold period past due, subsequently defaulted 60 days    
Allowance for credit loss $ 6,001   $ 6,440
Increase (decrease) in allowance for credit loss 100    
Performing Financial Instruments      
Troubled debt restructurings (TDRs) 700 $ 700  
TDRs | Nonperforming loans      
Troubled debt restructurings (TDRs) 800 900  
Federal Reserve Bank      
Loans pledged as collateral for the ability to borrow 25,600 30,100  
Federal Home Loan Bank      
Loans pledged as collateral for the ability to borrow $ 77,800 $ 69,000