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Borrowings - Narrative (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended |
--- | --- | --- | --- | ---
Mar. 16, 2022 | Mar. 31, 2020 | Mar. 31, 2022 | Mar. 31, 2022 | Dec. 31, 2021
Line of Credit Facility [Line Items] |  |  |  |  |
Total loans and accrued interest amount securitized at inception |  |  | $ 0 | $ 0 | $ 1,656,263,000
Vertical risk retention interest |  | 5.00% | 5.00% | 5.00% |
Uncommitted federal funds |  |  | $ 125,000,000 | $ 125,000,000 |
Lendable value of collateral |  |  | $ 2,900,000,000 | $ 2,900,000,000 | $ 3,300,000,000
2022-A |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Vertical risk retention interest | 5.00% |  |  |  |
2022-A | Student Loan |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Private education loans sold | $ 973,000,000 |  |  |  |
Additional net proceeds from sales of loans held for investment, capitalized interest | 95,000,000 |  |  |  |
Gain on sale | $ 10,000,000 |  |  |  |

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