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Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Receivables [Abstract] |  |  |
Average balance of Private Education Loans | $ 22,426,216 | $ 22,225,473 | $ 19,282,500
Average Balance, FFELP Loans | 757,953 | 814,198 | 888,301
Average Balance of Personal Loans | 582,552 | 1,141,503 | 900,152
Average Balance, Total portfolio | $ 23,766,721 | $ 24,181,174 | $ 21,070,953
Weighted Average Interest Rate, Private Education Loans | 8.42% | 9.32% | 9.10%
Weighted Average Interest Rate, FFELP Loans | 3.76% | 4.79% | 4.57%
Weighted Average Interest Rate of Personal Loans | 12.43% | 12.09% | 11.08%

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