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Borrowings - Additional Information (Details) - USD ($) |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 13, 2019 | Feb. 20, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Apr. 05, 2017
Line of Credit Facility [Line Items] |  |  |  |  |  |
Short-term borrowings |  |  | $ 0 | $ 0 |  |
Unsecured offering issued |  |  | $ 453,000,000 | $ 1,900,500,000 | $ 1,448,000,000 |
Estimated weighted average life of student loans (in years) |  |  | 5 years 5 months 5 days | 5 years 4 months 24 days |  |
Uncommitted federal funds |  |  | $ 125,000,000 |  |  |
Lendable value of collateral |  |  | 3,300,000,000 | $ 3,100,000,000 |  |
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 |  |  |
Private Education Loans 2018 Term A |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total loan amount securitized at inception | $ 453,000,000 |  |  |  |  |
Amount of loan securitization sold to third parties | $ 453,000,000 |  |  |  |  |
Ownership interest percentage | 100.00% |  |  |  |  |
Proceeds from loan securitization sold to third parties | $ 451,000,000 |  |  |  |  |
Loans pledged as collateral |  |  | 462,000,000 |  |  |
Class A and B Notes |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Estimated weighted average life of student loans (in years) | 4 years 3 months 2 days |  |  |  |  |
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 | 0.92% |  |  |  |  |
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 |  |  | $ 198,000,000 |  |  | $ 200,000,000
Interest rate stated percentage |  |  |  |  |  | 5.125%

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