Borrowings - Additional Information (Details) - USD ($)				9 Months Ended	12 Months Ended
	Jul. 21, 2016	May 26, 2016	Feb. 25, 2016	Sep. 30, 2016	Dec. 31, 2015	Dec. 19, 2014
Line of Credit Facility [Line Items]
Short-term borrowings				$ 350,000,000	$ 500,175,000
Estimated weighted average life of student loans (in years)				6 years	6 years 2 months 12 days
Uncommitted federal funds				$ 100,000,000
Lendable value of collateral				2,500,000,000	$ 1,700,000,000
ABCP borrowings
Line of Credit Facility [Line Items]
Short-term borrowings				350,000,000	$ 500,175,000
Loans pledged as collateral				$ 428,700,000
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 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%
Ownership interest percentage				100.00%
Private Education Loans
Line of Credit Facility [Line Items]
Total loan amount securitized at inception for the 2016 on-balance sheet term securitization	$ 657,000,000	$ 551,000,000
Proceeds from loan securitization sold to third parties	$ 607,000,000	$ 501,000,000
Class B Notes
Line of Credit Facility [Line Items]
Ownership interest percentage	100.00%	100.00%
Amount of ownership interest in loan securitization	$ 50,000,000	$ 50,000,000
Residual Certificates
Line of Credit Facility [Line Items]
Ownership interest percentage	100.00%	100.00%
Class A Notes
Line of Credit Facility [Line Items]
Amount of loan securitization sold to third parties	$ 607,000,000	$ 501,000,000
Proceeds from loan securitization sold to third parties	$ 607,000,000	$ 501,000,000
Estimated weighted average life of student loans (in years)	4 years 3 days	4 years 3 days
Private Education Loans Issuance 2016-A
Line of Credit Facility [Line Items]
Loans pledged as collateral				$ 571,000,000
Private Education Loans Issuance 2016-B
Line of Credit Facility [Line Items]
Loans pledged as collateral				$ 692,000,000
LIBOR | ABCP borrowings | Commercial Paper
Line of Credit Facility [Line Items]
Basis spread on variable rate			1.00%		0.80%
LIBOR | Class A Notes
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.36%	1.38%
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%