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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, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Accounts Notes And Loans Receivable [Line Items]        
Adjustment resulting from the change in the charge-off rate $ (1) $ 0 $ (2) $ 0
Expected future recoveries of current period defaults 13 10 23 21
Charge-offs (as a result of lower recovery expectations) (100) (87) (187) (207)
Change in balance during period (2) (6) (8) (15)
Private Education Loans [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Adjustment resulting from the change in the charge-off rate (1) 0 (2) 0
Expected future recoveries of current period defaults 13 10 23 21
Charge-offs (as a result of lower recovery expectations) (92) (77) (173) (187)
Change in balance during period (2) (6) (8) (15)
Private Education Loans [Member] | Loans Receivable For Previously Fully Charged Off Loans [Member]        
Accounts Notes And Loans Receivable [Line Items]        
Beginning of period expected future recoveries on previously fully charged-off loans 174 217 179 226
Expected future recoveries of current period defaults 13 10 23 21
Recoveries (cash collected) (11) (10) (21) (21)
Charge-offs (as a result of lower recovery expectations) (4) (6) (10) (15)
End of period expected future recoveries on previously fully charged-off loans 172 211 172 211
Change in balance during period $ (2) $ (6) $ (8) $ (15)