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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Receivables [Abstract] |  |  |
Average balance of Private Education Loans | $ 19,282,500 | $ 16,176,351 | $ 12,747,756
Average Balance, FFELP Loans | 888,301 | 970,738 | 1,063,325
Average Balance of Personal Loans | 900,152 | 112,644 | 710
Average Balance, Total portfolio | $ 21,070,953 | $ 17,259,733 | $ 13,811,791
Weighted Average Interest Rate, Private Education Loans | 9.10% | 8.43% | 8.02%
Weighted Average Interest Rate, FFELP Loans | 4.57% | 3.91% | 3.53%
Weighted Average Interest Rate of Personal Loans | 11.08% | 9.90% | 8.53%

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