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Allowance for Credit Losses and Reserve for Unfunded Lending Commitments - Summary of Loss Sharing Arrangements (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]    
Provision (benefit) for credit losses $ (823) $ 5,423
Loss sharing agreements    
Financing Receivable, Allowance for Credit Loss [Roll Forward]    
Expected reimbursement from loss sharing partners, beginning balance 2,159 2,166
Amounts due from partners which reduced net charge-offs (141) (302)
Provision (benefit) for credit losses (13) 789
Expected reimbursement from loss sharing partners, ending balance $ 2,005 $ 2,653