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Borrowings - Additional (Details) - USD ($) |  |  |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 19, 2018 | Jun. 20, 2018 | Mar. 21, 2018 | Feb. 21, 2018 | Nov. 08, 2017 | Feb. 08, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2018 | Apr. 05, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amount drawn and outstanding under ABCP facility |  |  |  |  |  |  | $ 0 | $ 0 |  |  |
Total Issued |  |  |  |  |  |  | 12,453,903,000 | 1,448,000,000 |  | $ 1,900,500,000 |
Unsecured debt issued |  |  |  |  |  |  | $ 0 | $ 197,000,000 | $ 0 |  |
Estimated weighted average life of student loans |  |  |  |  |  |  | 5 years 5 months 5 days | 5 years 6 months |  |  |
Discretionary uncommitted federal funds line of credit |  |  |  |  |  |  | $ 125,000,000 |  |  |  |
Lendable value of collateral |  |  |  |  |  |  | 3,100,000,000 | $ 2,600,000,000 |  |  |
ABCP borrowings |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amount drawn and outstanding under ABCP facility |  |  |  |  |  |  | $ 0 | 0 |  |  |
Contractual maturity related to ABCP facility |  |  |  |  |  |  | 2 years |  |  |  |
Revolving period of contractual maturity |  |  |  |  |  |  | 1 year |  |  |  |
Amortization period of contractual maturity |  |  |  |  |  |  | 1 year |  |  |  |
Private Education Loans 2018 Term A [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing |  |  | $ 670,000,000 |  |  |  |  |  |  |  |
Ownership interest percentage on asset-backed financing |  |  | 100.00% |  |  |  |  |  |  |  |
Loans sold to third parties |  |  | $ 670,000,000 |  |  |  |  |  |  |  |
Unsecured debt issued |  |  | $ 668,000,000 |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | $ 647,000,000 |  |  |  |
Private Education Loans 2018 Term B [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing |  | $ 687,000,000 |  |  |  |  |  |  |  |  |
Ownership interest percentage on asset-backed financing |  | 100.00% |  |  |  |  |  |  |  |  |
Loans sold to third parties |  | $ 687,000,000 |  |  |  |  |  |  |  |  |
Unsecured debt issued |  | $ 683,000,000 |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 678,000,000 |  |  |  |
Private Education Loans 2018 Term C [Member] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing | $ 544,000,000 |  |  |  |  |  |  |  |  |  |
Ownership interest percentage on asset-backed financing | 100.00% |  |  |  |  |  |  |  |  |  |
Loans sold to third parties | $ 544,000,000 |  |  |  |  |  |  |  |  |  |
Unsecured debt issued | $ 541,000,000 |  |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 544,000,000 |  |  |  |
Private Education Loans |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing |  |  |  |  | $ 676,000,000 | $ 772,000,000 | 1,900,000,000 |  |  |  |
Ownership interest percentage on asset-backed financing |  |  |  |  | 100.00% | 100.00% |  |  |  |  |
Unsecured debt issued |  |  |  |  | $ 674,000,000 | $ 768,000,000 |  |  |  |  |
Private Education Loans, 2017 Term B |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 617,000,000 |  |  |  |
ABS Transactions |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan term accounted for as a secured financing |  |  |  |  |  |  |  |  | $ 2,400,000,000 |  |
Private Education Loans, 2017 Term A |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  |  |  |  |  |  | 631,000,000 |  |  |  |
Class A and B Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Estimated weighted average life of student loans | 4 years 3 months 26 days | 4 years 4 months 25 days | 4 years 5 months 4 days |  | 4 years 26 days | 4 years 3 months 8 days |  |  |  |  |
London Interbank Offered Rate (LIBOR) | Class A and B Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 0.77% | 0.76% | 0.78% |  | 0.80% | 0.93% |  |  |  |  |
Commercial Paper | ABCP borrowings |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Private education loan funding |  |  |  | $ 750,000,000.0 |  |  |  |  |  |  |
Ownership Interest Percentage |  |  |  | 100.00% |  |  |  |  |  |  |
Amount drawn and outstanding under ABCP facility |  |  |  |  |  |  | 0 | 0 |  |  |
Commercial Paper | London Interbank Offered Rate (LIBOR) | ABCP borrowings |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.85% |  |  |  |  |  |  |
Minimum | Commercial Paper | ABCP borrowings |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unused borrowing capacity fee (as a percent) |  |  |  | 0.35% |  |  |  |  |  |  |
Maximum | Commercial Paper | ABCP borrowings |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unused borrowing capacity fee (as a percent) |  |  |  | 0.45% |  |  |  |  |  |  |
Unsecured Debt |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amount drawn and outstanding under ABCP facility |  |  |  |  |  |  | 0 | $ 0 |  |  |
Total Issued |  |  |  |  |  |  | 200,000,000 |  |  |  |
Unsecured Debt | Senior Unsecured Notes Due April 5, 2022 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total Issued |  |  |  |  |  |  | $ 197,000,000 |  |  |  | $ 200,000,000
Interest rate |  |  |  |  |  |  |  |  |  |  | 5.125%

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