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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, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
Receivables [Abstract] |  |  |  |
Average Balance, Private Education Loans | $ 12,217,890 | $ 9,361,711 | $ 12,017,799 | $ 9,407,888
Average Balance FFELP Loans | 1,076,419 | 1,194,309 | 1,089,836 | 1,214,384
Average balance, Total portfolio | $ 13,294,309 | $ 10,556,020 | $ 13,107,635 | $ 10,622,272
Weighted Average Interest Rate, Private Education Loans | 7.98% | 7.96% | 8.00% | 8.01%
Weighted Average Interest Rate FFELP loans | 3.48% | 3.22% | 3.45% | 3.20%

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