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Borrowings - Narrative (Details) - USD ($) $ in Millions |  | 3 Months Ended |
--- | --- | --- | ---
Mar. 13, 2024 | Mar. 31, 2024 | Dec. 31, 2023
Line of Credit Facility [Line Items] |  |  |
Vertical risk retention interest |  | 5.00% |
Uncommitted federal funds |  | $ 125 |
Lendable value of collateral |  | $ 1,400 | $ 1,600
2023-B |  |  |
Line of Credit Facility [Line Items] |  |  |
Vertical risk retention interest |  | 5.00% |
2023-B | Private Education Loans |  |  |
Line of Credit Facility [Line Items] |  |  |
Private education loans sold | $ 2,000 |  |
Additional proceeds from sale | 105 |  |
Gain on sale | $ 7 |  |

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