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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Receivables [Abstract] |  |
Average Balance, Private Education Loans | $ 11,817,708 | $ 9,454,579
Average Balance FFELP Loans | 1,103,253 | 1,234,682
Average balance, Total portfolio | $ 12,920,961 | $ 10,689,261
Weighted Average Interest Rate, Private Education Loans | 8.03% | 8.07%
Weighted Average Interest Rate FFELP loans | 3.42% | 3.19%

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