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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Average Balance | $ 20,108,078 |  | $ 16,488,513
Private Education Loans |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Average Balance | $ 18,659,717 |  | 15,449,555
Weighted Average Interest Rate | 8.84% | 8.26% |
FFELP Loans |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Average Balance | $ 919,717 |  | 1,003,128
Weighted Average Interest Rate | 4.25% | 3.69% |
Personal Loans |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Average Balance | $ 528,644 |  | $ 35,830
Weighted Average Interest Rate | 10.64% | 9.16% |

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