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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, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 20,478,219 | $ 16,729,191 | $ 20,294,171 | $ 16,609,541
Private Education Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 18,764,768 | $ 15,687,803 | $ 18,712,533 | $ 15,569,337
Weighted Average Interest Rate | 9.03% | 8.33% | 8.93% | 8.30%
FFELP Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 898,095 | $ 980,478 | $ 908,846 | $ 991,740
Weighted Average Interest Rate | 4.51% | 3.87% | 4.38% | 3.78%
Personal Loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Average Balance | $ 815,356 | $ 60,910 | $ 672,792 | $ 48,464
Weighted Average Interest Rate | 10.65% | 9.28% | 10.65% | 9.24%

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