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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Receivables [Abstract] |  |  |  |
Average Balance, Private Education Loans | $ 15,687,803 | $ 12,217,890 | $ 15,569,337 | $ 12,017,799
Average Balance FFELP Loans | 980,478 | 1,076,419 | 991,740 | 1,089,836
Average Balance of Personal Loans | 60,910 | 0 | 48,894 | 0
Average balance, Total portfolio | $ 16,729,191 | $ 13,294,309 | $ 16,609,971 | $ 13,107,635
Weighted Average Interest Rate, Private Education Loans | 8.33% | 7.98% | 8.30% | 8.00%
Weighted Average Interest Rate FFELP loans | 3.87% | 3.48% | 3.78% | 3.45%
Weighted Average Interest Rate of Personal Loans | 9.28% | 0.00% | 9.19% |

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