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Bonds and Notes Payable - Narrative (Details) - USD ($) |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 07, 2019 | May 30, 2019 | Jan. 11, 2019 | Sep. 30, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Apr. 25, 2019 | Jan. 01, 2019 | Dec. 31, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Payment of premium to extinguish debt |  |  |  |  | $ 14,030,000 | $ 0 |  |  |
Loss on extinguishment of debt |  |  |  |  | 15,815,000 | $ 0 |  |  |
Unsecured Line of Credit |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Maximum financing amount |  |  |  | $ 382,500,000 | 382,500,000 |  |  |  |
Amount outstanding |  |  |  | 0 | 0 |  |  |  |
Higher borrowing capacity option |  |  |  | 400,000,000.0 | 400,000,000.0 |  |  |  |
FFELP Asset-Backed Security Investment |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Other borrowings subject to repurchase agreement |  |  |  | 40,500,000 | 40,500,000 |  |  |  | $ 41,400,000
Private education loans |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Other borrowings subject to repurchase agreement |  |  |  |  |  |  |  |  | $ 45,000,000.0
Secured Line Of Credit Maturing May 2022 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Maximum financing amount |  | $ 22,000,000.0 |  |  |  |  |  |  |
Amount outstanding |  |  |  | 5,000,000.0 | 5,000,000.0 |  |  |  |
Amount available |  |  |  | 17,000,000.0 | 17,000,000.0 |  |  |  |
Secured Line Of Credit Maturing May 2022 | LIBOR |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Spread on variable interest rate |  | 1.75% |  |  |  |  |  |  |
Asset-backed securities |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Amount of debt extinguished | $ 93,000,000.0 |  |  | 675,600,000 |  |  |  |  |
Payment of premium to extinguish debt | 1,400,000 |  |  | 12,600,000 |  |  |  |  |
Write off of debt issuance costs | 400,000 |  |  | 1,400,000 |  |  |  |  |
Loss on extinguishment of debt | $ 1,800,000 |  |  | 14,000,000.0 |  |  |  |  |
Warehouse facilities | Consumer Loan Warehouse Facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Maximum financing amount |  |  | $ 100,000,000.0 |  |  |  | $ 200,000,000.0 |  |
Amount outstanding |  |  |  | 144,000,000.0 | 144,000,000.0 |  |  |  |
Amount available |  |  |  | 56,000,000.0 | 56,000,000.0 |  |  |  |
Advanced as equity support |  |  |  | $ 50,200,000 | $ 50,200,000 |  |  |  |
Minimum | Warehouse facilities | Consumer Loan Warehouse Facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Advance rate |  |  | 70.00% |  |  |  |  |  |
Maximum | Warehouse facilities | Consumer Loan Warehouse Facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Advance rate |  |  | 75.00% |  |  |  |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Issue price |  |  |  |  |  |  |  | $ 33,900,000 |
Partnerships For Certain Real Estate Development Projects |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Noncontrolling interest ownership percentage |  |  |  | 50.00% | 50.00% |  |  |  |

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