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Loans Held for Investment - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Jun. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Percent of private loans indexed to LIBOR | 69.00% |  |  | 69.00% |  | 77.00% |  |  |
Gain on sale of portfolio split loans | $ 0 |  | $ 0 | $ 2,060 | $ 0 |  |  |  |
Tier 1 of government guarantee (in percentage) | 97.00% |  |  | 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 4 months 24 days |  | 5 years 6 months |  |  |
Discontinued Operations, Disposed of by Sale |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Portfolio split loans sold |  | $ 43,000 |  |  |  |  |  |  |
Gain on sale of portfolio split loans |  | $ 2,000 |  |  |  |  |  |  |

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- Definition Reflects the net positive or negative amount derived from subtracting from net proceeds of sale and the carrying amounts, net of allocated reserves, of financial assets transferred to third parties in transactions that qualify for sales treatment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107425-111719 + Details
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- Definition This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719 + Details
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