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Allowance for Loan Losses - Amount of Loans Modified for Borrowers Financial Difficulty and Payment Default (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Payment Default (Par) |  | $ 46 | $ 21
Private Education Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Modified Loans (Amortized Cost) | $ 272 |  |
Payment Default (Par) | $ 278 |  |

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