Borrowings - Additional Information (Details) - USD ($)				3 Months Ended	12 Months Ended
	Feb. 22, 2017	Feb. 08, 2017	Feb. 25, 2016	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Short-term borrowings				$ 0	$ 0
Estimated weighted average life of student loans (in years)				6 years	6 years
Uncommitted federal funds				$ 125,000,000
Lendable value of collateral				2,600,000,000	$ 2,600,000,000
ABCP borrowings
Line of Credit Facility [Line Items]
Short-term borrowings				$ 0	$ 0
ABCP borrowings | Commercial Paper
Line of Credit Facility [Line Items]
Private asset backed commercial paper education loan funding facility	$ 750,000,000		$ 750,000,000.0
Ownership interest percentage in residual interest in ABCP facility	100.00%			100.00%
Short-term borrowings				$ 0
Private Education Loans
Line of Credit Facility [Line Items]
Total loan amount securitized at inception		$ 772,000,000
Amount of loan securitization sold to third parties		$ 772,000,000
Class A Notes
Line of Credit Facility [Line Items]
Ownership interest percentage		100.00%
Proceeds from loan securitization sold to third parties		$ 768,000,000
Class A and B Notes
Line of Credit Facility [Line Items]
Estimated weighted average life of student loans (in years)		4 years 3 months 7 days
Private Education Loans, 2016 Term A
Line of Credit Facility [Line Items]
Loans pledged as collateral				$ 795,000,000
LIBOR | ABCP borrowings | Commercial Paper
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.90%		0.90%
LIBOR | Class A and B Notes
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.93%
Minimum | ABCP borrowings | Commercial Paper
Line of Credit Facility [Line Items]
Financing cost percentage of unused borrowing capacity	0.35%		0.35%
Maximum | ABCP borrowings | Commercial Paper
Line of Credit Facility [Line Items]
Financing cost percentage of unused borrowing capacity	0.45%		0.45%