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Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Accounts Notes And Loans Receivable [Line Items]        
Sale of private education loan $ 30   $ 1,600  
End of period expected recoveries 434 $ 549 434 $ 549
Change in balance during period (20) (22) (45) (39)
Private Education Loans [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Beginning of period expected recoveries 454 571 479 588
Expected future recoveries of current period defaults 5 9 10 22
Recoveries (22) (28) (47) (57)
Charge-offs (3) (3) (8) (4)
End of period expected recoveries 434 549 434 549
Change in balance during period $ (20) $ (22) $ (45) $ (39)