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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 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]        
Amounts estimated to be charged to partners which reduced provision for credit losses $ 4,246 $ 1,342 $ 9,669 $ 3,035
Loss sharing agreements        
Financing Receivable, Allowance for Credit Loss [Roll Forward]        
Expected reimbursement from loss sharing partners, beginning balance 2,653 442 2,166 379
Amounts due from partners which reduced net charge-offs (293) (105) (595) (213)
Amounts estimated to be charged to partners which reduced provision for credit losses 73 77 862 248
Expected reimbursement from loss sharing partners, ending balance $ 2,433 $ 414 $ 2,433 $ 414