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Allowance for Credit Losses and Reserve for Unfunded Lending Commitments - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for credit loss $ 15,564 $ 7,208    
Uncollectible Portion of Billed Finance Charges and Fees 1,100 1,400 $ 1,300  
Allowance for credit losses        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Increase (Decrease) in Allowance for Credit Loss 8,400      
Allowance for credit loss $ 15,564 $ 7,208 $ 7,220 $ 7,502