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Loans Receivable and Allowance for Loan Losses - Loans Receivable (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 3 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
May 01, 2019 | Nov. 07, 2019 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | $ 21,206,948 |  | $ 22,520,498 |  |  |
Loan discount, net of unamortized loan premiums and deferred origination costs, and non-accretable discount |  |  |  | (36,483) |  | (53,572) |  |  |
Allowance for loan losses |  |  |  | (66,417) | $ (62,591) | (60,388) | $ (60,217) | $ (53,715) | $ (54,590)
Loans receivable |  |  |  | 21,071,441 |  | 22,377,142 |  |  |
Non-accretable discount |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loan discount, net of unamortized loan premiums and deferred origination costs, and non-accretable discount |  |  |  | (32,607) |  | (29,396) |  |  |
Federally insured loans |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | 20,695,837 |  | 22,155,896 |  |  |
Allowance for loan losses |  |  |  | (37,676) | (39,056) | (42,310) | (43,053) | (37,263) | (38,706)
Federally insured loans | Stafford and other |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | 4,720,338 |  | 4,969,667 |  |  |
Federally insured loans | Consolidation |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | 15,975,499 |  | 17,186,229 |  |  |
Private education loans |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | 189,912 |  | 225,975 |  |  |
Allowance for loan losses |  |  |  | (9,882) | (10,157) | (10,838) | (11,253) | (11,664) | (12,629)
Consumer loans |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans receivable, gross |  |  |  | 321,199 |  | 138,627 |  |  |
Allowance for loan losses |  |  |  | $ (18,859) | $ (13,378) | $ (7,240) | $ (5,911) | $ (4,788) | $ (3,255)
Consumer loans | Financing receivable |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans sold (par value) | $ 47,700 |  |  |  |  |  |  |  |
Gain on sale of loan | $ 1,700 |  |  |  |  |  |  |  |
Beneficial interest received as partial consideration | 11.00% |  |  |  |  |  |  |  |
Consumer loans | Financing receivable | Forecast |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Gain on sale of loan |  |  | $ 15,500 |  |  |  |  |  |
Consumer loans | Financing receivable | Subsequent event |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |
Loans sold (par value) |  | $ 179,300 |  |  |  |  |  |  |
Beneficial interest received as partial consideration |  | 29.00% |  |  |  |  |  |  |

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