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Loans Held for Investment - Additional Information (Detail) | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) payment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Private loans indexed to LIBOR (as a percentage) |  | 67.00% | 77.00% |  |  |  |
Gains on sales of loans, net |  | $ 2,060,000 | $ 0 | $ 230,000 |  |  |
Tier 1 of government guarantee |  |  |  |  | 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 5 months 5 days | 5 years 6 months |  |  |  |
Number of payments to qualify for Private Education Loan program | payment |  | 3 |  |  |  |  |
Interest Rate Reduction, Percentage |  | $ 0.02 |  |  |  |  |
Period of loans past due that have accrued interest |  | 90 days |  |  |  |  |
Loans in nonaccrual status |  | $ 0 | $ 0 |  |  |  |
Percentage of loans concentrated in major states |  | 39.90% | 40.40% |  |  |  |
Percentage of aggregate outstanding loans held for investment |  | 5.00% |  |  |  |  |
FFELP Loans |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Ninety or more days delinquent |  | $ 42,900,000 | $ 66,900,000 |  |  |  |
Private Education Loans |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Ninety or more days delinquent |  | 50,800,000 | 37,600,000 |  |  |  |
Private Education Loans |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Loans pledged to borrower in custody |  | $ 3,400,000,000 | $ 2,700,000,000 |  |  |  |
Discontinued Operations, Disposed of by Sale |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Split Loans portfolio sales price | $ 43,000,000 |  |  |  |  |  |
Gains on sales of loans, net | $ 2,000,000 |  |  |  |  |  |

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