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Allowance for Credit Losses and Reserve for Unfunded Lending Commitments - Summary of Loss Sharing Arrangements (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]        
Amounts estimated to be charged to partners which reduced provision for credit losses $ 331 $ 1,383 $ 10,000 $ 4,418
Loss sharing agreements        
Financing Receivable, Allowance for Credit Loss [Roll Forward]        
Expected reimbursement from loss sharing partners, beginning balance 2,433 414 2,166 379
Amounts due from partners which reduced net charge-offs (212) (100) (807) (313)
Amounts estimated to be charged to partners which reduced provision for credit losses 121 86 983 334
Expected reimbursement from loss sharing partners, ending balance $ 2,342 $ 400 $ 2,342 $ 400