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Borrowings - Additional Information (Details) - USD ($) |  |  |  | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 20, 2018 | Mar. 21, 2018 | Feb. 21, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Apr. 05, 2017 | Dec. 31, 2016
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Short-term borrowings |  |  |  | $ 0 |  | $ 0 |  |
Unsecured offering issued |  |  |  | 1,356,500,000 |  | $ 1,448,000,000 |  | $ 1,782,000,000
Proceeds from loan securitization sold to third parties |  |  |  | $ 0 | $ 197,000,000 |  |  |
Estimated weighted average life of student loans (in years) |  |  |  | 5 years 3 months 5 days |  | 5 years 6 months 5 days |  |
Uncommitted federal funds |  |  |  | $ 125,000,000 |  |  |  |
Lendable value of collateral |  |  |  | 2,500,000,000 |  | $ 2,600,000,000 |  |
ABCP borrowings | Commercial Paper |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Private asset backed commercial paper education loan funding facility |  |  | $ 750,000,000 |  |  |  |  |
Ownership interest percentage in residual interest in ABCP facility |  |  | 100.00% |  |  |  |  |
Short-term borrowings |  |  |  | 0 |  | 0 |  |
Private Education Loans 2018 Term A |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Total loan amount securitized at inception |  | $ 670,000,000 |  |  |  |  |  |
Amount of loan securitization sold to third parties |  | $ 670,000,000 |  |  |  |  |  |
Ownership interest percentage |  | 100.00% |  |  |  |  |  |
Proceeds from loan securitization sold to third parties |  | $ 668,000,000 |  |  |  |  |  |
Loans pledged as collateral |  |  |  | 681,000,000 |  |  |  |
Class A and B Notes |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Estimated weighted average life of student loans (in years) | 4 years 4 months 25 days | 4 years 5 months 5 days |  |  |  |  |  |
Private Education Loans 2018 Term B |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Total loan amount securitized at inception | $ 687,000,000 |  |  |  |  |  |  |
Amount of loan securitization sold to third parties | $ 687,000,000 |  |  |  |  |  |  |
Ownership interest percentage | 100.00% |  |  |  |  |  |  |
Proceeds from loan securitization sold to third parties | $ 683,000,000 |  |  |  |  |  |  |
Loans pledged as collateral | $ 711,000,000 |  |  |  |  |  |  |
LIBOR | ABCP borrowings | Commercial Paper |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 0.85% |  |  |  |  |
LIBOR | Class A and B Notes |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate | 0.76% | 0.78% |  |  |  |  |  |
Minimum | ABCP borrowings | Commercial Paper |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Financing cost percentage of unused borrowing capacity |  |  | 0.35% |  |  |  |  |
Maximum | ABCP borrowings | Commercial Paper |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Financing cost percentage of unused borrowing capacity |  |  | 0.45% |  |  |  |  |
Unsecured borrowings |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Short-term borrowings |  |  |  | 0 |  | $ 0 |  |
Unsecured borrowings | Senior Unsecured Notes Due April 5, 2022 |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Unsecured offering issued |  |  |  | $ 197,000,000 |  |  | $ 200,000,000 |
Interest rate stated percentage |  |  |  |  |  |  | 5.125% |

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