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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, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Receivables [Abstract] |  |  |  |
Average Balance, Private Education Loans | $ 12,881,890 | $ 9,869,025 | $ 12,307,932 | $ 9,563,290
Average Balance FFELP Loans | 1,049,803 | 1,161,288 | 1,076,394 | 1,196,491
Average balance, Total portfolio | $ 13,931,693 | $ 11,030,313 | $ 13,384,326 | $ 10,759,781
Weighted Average Interest Rate, Private Education Loans | 8.00% | 7.87% | 8.00% | 7.96%
Weighted Average Interest Rate FFELP loans | 3.52% | 3.27% | 3.48% | 3.22%

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