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Allowance for Loan Losses - Amortized Cost Basis of Loans Modified to Borrowers with Financial Difficulty (Parenthetical) (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Accounts Notes And Loans Receivable [Line Items] |  |
Loans | $ 54,827 | $ 62,250
Private Education Loans [Member] |  |
Accounts Notes And Loans Receivable [Line Items] |  |
Loans | 16,902 | $ 18,725
Interest Rate Reduction [Member] | Private Education Loans [Member] |  |
Accounts Notes And Loans Receivable [Line Items] |  |
Loans | $ 1,200 |

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