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Allowance for Loan Losses - Amount of Loans Modified for Borrowers Financial Difficulty and Payment Default (Details) - Private Education Loans [Member] - 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] |  |  |  |
Modified Loans (Amortized Cost) | $ 109 | $ 128 | $ 179 | $ 163
Payment Default (Par) | 111 | 131 | 183 | 167
Charge-offs (par) | $ 15 | $ 10 | $ 25 | $ 12

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