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Borrowings - Additional Information (Details) - USD ($) |  |  |  | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Aug. 12, 2020 | Feb. 19, 2020 | Feb. 12, 2020 | Sep. 30, 2020 | Dec. 31, 2019 | Feb. 20, 2019
Line of Credit Facility [Line Items] |  |  |  |  |  |
Short-term borrowings |  |  |  | $ 0 | $ 289,230,000 |
Unsecured offering issued |  |  |  | $ 1,343,000,000 | $ 1,110,000,000 |
Estimated weighted average life of student loans |  |  |  | 5 years 4 months 24 days | 5 years 4 months 24 days |
Uncommitted federal funds |  |  |  | $ 125,000,000 |  |
Lendable value of collateral |  |  |  | 3,700,000,000 | $ 3,200,000,000 |
Unsecured borrowings |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Short-term borrowings |  |  |  | 0 | 0 |
Private Education Loans 2020 Term A |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total loan amount securitized at inception |  |  | $ 636,000,000 |  |  |
Amount of loan securitization sold to third parties |  |  | $ 636,000,000 |  |  |
Ownership interest percentage |  |  | 100.00% |  |  |
Proceeds from loan securitization sold to third parties |  |  | $ 634,000,000 |  |  |
Loans pledged as collateral |  |  |  | 626,000,000 |  |
Principal amount of collateral |  |  |  | 582,000,000 |  |
Capitalized interest |  |  |  | 44,000,000 |  |
Class A and B Notes |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Estimated weighted average life of student loans | 4 years 1 month 20 days |  | 4 years 2 months 4 days |  |  |
Class A and B Notes | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Basis spread on variable rate | 1.30% |  | 0.88% |  |  |
Private Education Loans 2020 Term B |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total loan amount securitized at inception | $ 707,000,000 |  |  |  |  |
Amount of loan securitization sold to third parties | $ 707,000,000 |  |  |  |  |
Ownership interest percentage | 100.00% |  |  |  |  |
Proceeds from loan securitization sold to third parties | $ 705,000,000 |  |  |  |  |
Loans pledged as collateral |  |  |  | 775,000,000 |  |
Principal amount of collateral |  |  |  | 720,000,000 |  |
Capitalized interest |  |  |  | 55,000,000 |  |
Commercial Paper | ABCP borrowings |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Private asset backed commercial paper education loan funding facility |  | $ 2,000,000,000 |  |  |  | $ 750,000,000
Ownership interest percentage in residual interest in ABCP facility |  | 100.00% |  |  |  |
Short-term borrowings |  |  |  | $ 0 | $ 289,000,000 |

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