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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Receivables [Abstract] |  |  |  |
Average Balance, Private Education Loans | $ 9,361,711 | $ 7,357,599 | $ 9,407,888 | $ 7,388,484
Average Balance FFELP Loans | 1,194,309 | 1,378,206 | 1,214,384 | 1,391,327
Average balance, total portfolio | $ 10,556,020 | $ 8,735,805 | $ 10,622,272 | $ 8,779,811
Weighted Average Interest Rate, Private Education Loans | 7.96% | 8.22% | 8.01% | 8.18%
Weighted Average Interest Rate FFELP loans | 3.22% | 3.32% | 3.20% | 3.26%

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