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%