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Significant Accounting Policies - Trading Investments (Details) - USD ($) $ in Billions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Vertical risk retention interest | 500.00% | 5.00%
Student Loan |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private education loans sold | $ 1.7 |

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