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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Receivables [Abstract] |  |  |  |
Average Balance, Private Education Loans | $ 16,228,751 | $ 12,881,890 | $ 15,791,557 | $ 12,307,932
Average Balance FFELP Loans | 960,185 | 1,049,803 | 981,106 | 1,076,394
Average Balance of Personal Loans | 86,441 | 0 | 61,263 | 0
Average balance, Total portfolio | $ 17,275,377 | $ 13,931,693 | $ 16,833,926 | $ 13,384,326
Weighted Average Interest Rate, Private Education Loans | 8.50% | 8.00% | 8.37% | 8.00%
Weighted Average Interest Rate FFELP loans | 4.02% | 3.52% | 3.86% | 3.48%
Weighted Average Interest Rate of Personal Loans | 9.66% | 0.00% | 9.44% | 0.00%

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