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Loans Held for Investment - Additional Information (Details) - USD ($) | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jul. 01, 2006 | Jun. 30, 2006 | Sep. 30, 1993
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Private loans indexed to LIBOR |  | 58.00% | 67.00% |  |  |  |
Gains on sales of loans, net |  | $ 0 | $ 2,060,000 | $ 0 |  |  |
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 4 months 24 days | 5 years 5 months |  |  |  |
Loans currently in repayment status that were subject to interest rate reductions |  | 7.20% | 6.40% |  |  |  |
Period of loans past due that have accrued interest |  | 90 days |  |  |  |  |
Loans in nonaccrual status |  | $ 0 | $ 0 |  |  |  |
Percentage of loans concentrated in major states |  | 39.40% | 39.90% |  |  |  |
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 |  | $ 48,000,000 | $ 43,000,000 |  |  |  |
Private Education Loans |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Ninety or more days delinquent |  | 63,000,000 | 51,000,000 |  |  |  |
Private Education Loans |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Loans pledged to borrower in custody |  | $ 3,400,000,000 | $ 3,400,000,000 |  |  |  |
Maximum |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Interest rate reduction |  | 4.00% |  |  |  |  |
Increase to life of loan defaults |  | 14.00% |  |  |  |  |
Minimum |  |  |  |  |  |  |
Finance Receivable Transferred To Held For Sale [Line Items] |  |  |  |  |  |  |
Interest rate reduction |  | 2.00% |  |  |  |  |
Increase to life of loan defaults |  | 4.00% |  |  |  |  |
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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