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Borrowings - Additional Information (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 25, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Dec. 19, 2014
Line of Credit Facility [Line Items] |  |  |  |
Short-term borrowings |  | $ 526,500,000 | $ 500,175,000 |
Uncommitted federal funds |  | 100,000,000 |  |
Lendable value of collateral |  | 1,500,000,000 | 1,700,000,000 |
ABCP borrowings |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Short-term borrowings |  | 526,500,000 | $ 500,175,000 |
Loans pledged as collateral |  | $ 902,000,000 |  |
Contractual maturity |  | 2 years |  |
Contractual maturity term, revolving period |  | 1 year |  |
Contractual maturity term, amortization period |  | 1 year |  |
ABCP borrowings | Commercial Paper |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Private asset backed commercial paper education loan funding facility | $ 750,000,000.0 |  |  | $ 750,000,000.0
Ownership Interest Percentage |  | 100.00% |  |
Ownership interest percentage in loan facility |  |  |  | 5.00%
Required ownership interest |  |  |  | $ 37,500,000
Funds available for Private Education Loan originations |  |  |  | $ 712,500,000
Financing cost percentage of unused borrowing capacity |  |  | 0.40% |
LIBOR | ABCP borrowings | Commercial Paper |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Basis spread on variable rate | 1.00% |  | 0.80% |
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% |  |  |

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