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Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Parenthetical) (Details) - Private Education Loans [Member] - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024 | Jun. 30, 2024
--- | --- | --- | ---
Accounts Notes And Loans Receivable [Line Items] |  |  |
Loans | $ 15,530 | $ 15,716 | $ 16,238
Interest Rate Concession [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Loans | $ 1,000 |  | $ 1,100

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