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Loans Held for Investment - Additional Information (Detail) | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Receivables [Abstract] |  |  |  |  |  |
Percent of private loans indexed to LIBOR | 74.00% |  | 77.00% |  |  |
Tier 1 of government guarantee (in percentage) | 97.00% |  |  | 97.00% |  |
Tier 2 of government guarantee (in percentage) |  |  |  |  | 98.00% |
Tier 3 of government guarantee (in percentage) |  |  |  |  |  | 100.00%
Estimated weighted average life of student loans (in years) | 5 years 5 months 5 days | 5 years 6 months 5 days |  |  |  |

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- Definition Percent of Private Loans indexed to LIBOR + References No definition available. + Details
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- Definition Tier One of Government Guarantee + References No definition available. + Details
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- Definition Tier Three of Government Guarantee + References No definition available. + Details
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