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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Receivables [Abstract] |  |  |
Average balance of Private Education Loans | $ 16,176,351 | $ 12,747,756 | $ 9,819,053
Average Balance, FFELP Loans | 970,738 | 1,063,325 | 1,179,723
Average Balance of Personal Loans | 112,644 | 710 | 0
Average Balance, Total portfolio | $ 17,259,733 | $ 13,811,791 | $ 10,998,776
Weighted Average Interest Rate, Private Education Loans | 8.43% | 8.02% | 7.93%
Weighted Average Interest Rate, FFELP Loans | 3.91% | 3.53% | 3.26%
Weighted Average Interest Rate of Personal Loans | 9.90% | 8.53% | 0.00%

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