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Loans Held for Investment - Additional Information (Details) | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018 | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Percent of private loans indexed to LIBOR | 59.00% | 67.00% |  |  |
Tier 1 of government guarantee | 97.00% |  | 97.00% |  |
Tier 2 of government guarantee |  |  |  | 98.00% |
Tier 3 of government guarantee |  |  |  |  | 100.00%
Estimated weighted average life of student loans | 5 years 4 months 24 days | 5 years 4 months 24 days |  |  |
Interest rate offered to borrowers facing financial difficulty | 4.00% |  |  |  |
Interest rate offered to borrowers facing financial difficulty, period | 2 years |  |  |  |
Minimum |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Interest rate offered to borrowers facing financial difficulty | 2.00% |  |  |  |
Maximum |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Interest rate offered to borrowers facing financial difficulty | 4.00% |  |  |  |

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