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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 21,254,774 | $ 17,275,377 | $ 20,617,890 | $ 16,833,926
Private Education Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 19,295,318 | $ 16,228,751 | $ 18,908,929 | $ 15,791,557
Weighted Average Interest Rate | 9.16% | 8.50% | 9.01% | 8.37%
FFELP Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 877,279 | $ 960,185 | $ 898,208 | $ 981,106
Weighted Average Interest Rate | 4.65% | 4.02% | 4.47% | 3.86%
Personal Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 1,082,177 | $ 86,441 | $ 810,753 | $ 61,263
Weighted Average Interest Rate | 11.03% | 9.66% | 10.82% | 9.44%

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