Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	2,837,347	2,167,979
Total	2,837,347	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	94,146	53,717
Other Assets	47,855	52,153
Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	2,837,347	2,167,979
Total	2,837,347	2,167,979
ABCP Facility
Debt Outstanding
Short Term	0	0
Long Term	0	0
Total	0	0
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	2,837,347	2,167,979
Total	2,837,347	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,275,417	2,562,156
Restricted Cash	90,682	44,617
Other Assets	217,687	160,783
Total	3,583,786	2,767,556
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	2,837,347	2,167,979
Total	2,837,347	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,275,417	2,562,156
Restricted Cash	90,682	44,617
Other Assets	217,687	160,783
Total	3,583,786	2,767,556
Variable Interest Entity, Primary Beneficiary | ABCP Facility
Debt Outstanding
Short Term	0	0
Long Term	0	0
Total	0	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	0	0
Restricted Cash	0	0
Other Assets	0	0
Total	$ 0	$ 0