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Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class and Company's Internal Credit Quality Rating (Parenthetical) (Detail) - Government National Mortgage Association [Member] - USD ($)
$ in Billions
Sep. 30, 2020
Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]    
Restructured GNMA loans,classified with a pass rating $ 1.6 $ 1.7
GNMA loans 90 days or more past due, classified with a pass rating $ 1.4 $ 1.6