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Bonds and Notes Payable - Outstanding Debt Obligations (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Jan. 01, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 20,910,190 | $ 22,185,558 | $ 22,218,740
Discount on bonds and notes payable and debt issuance costs | (284,468) |  | (318,359)
Warehouse facilities | FFELP warehouse facilities |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 739,448 |  | 986,886
Unsecured line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 0 |  | 310,000
Unsecured debt - Junior Subordinated Hybrid Securities |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 20,381 |  | $ 20,381
Interest rate range | 5.46% |  | 6.17%
Other borrowings |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 45,465 |  | $ 120,342
Bonds and notes payable, gross |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 21,194,658 |  | 22,537,099
Federally insured | Bonds and notes based on indices |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 18,961,410 |  | 20,192,123
Federally insured | Bonds and notes based on auction |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 773,726 |  | 793,476
Federally insured | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 374,500 |  |
Federally insured | Variable-rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | 19,735,136 |  | 20,985,599
Consumer loan | Warehouse facilities |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 144,022 |  |
Interest rate range | 2.23% |  |
Private education | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 53,368 |  | 63,171
Private education | Variable-rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Bonds and notes payable | $ 82,338 |  | $ 50,720
Interest rate range |  |  | 4.26%
Minimum | Warehouse facilities | FFELP warehouse facilities |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 2.22% |  | 2.65%
Minimum | Unsecured line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range |  |  | 3.92%
Minimum | Other borrowings |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 2.77% |  | 3.05%
Minimum | Federally insured | Bonds and notes based on indices |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 2.20% |  | 2.59%
Minimum | Federally insured | Bonds and notes based on auction |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.02% |  | 2.84%
Minimum | Federally insured | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 2.53% |  |
Minimum | Private education | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.60% |  | 3.60%
Minimum | Private education | Variable-rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.40% |  |
Maximum | Warehouse facilities | FFELP warehouse facilities |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 2.26% |  | 2.71%
Maximum | Unsecured line of credit |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range |  |  | 4.01%
Maximum | Other borrowings |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.85% |  | 5.22%
Maximum | Federally insured | Bonds and notes based on indices |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.81% |  | 4.52%
Maximum | Federally insured | Bonds and notes based on auction |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.84% |  | 3.55%
Maximum | Federally insured | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.45% |  |
Maximum | Private education | Fixed rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 5.35% |  | 5.35%
Maximum | Private education | Variable-rate bonds and notes |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate range | 3.77% |  |

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