Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	3,744,345	3,275,270
Total	3,744,345	3,275,270
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	120,084	101,836
Other Assets	93,332	84,853
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	3,547,604	3,078,731
Total	3,547,604	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	4,243,820	3,691,024
Restricted Cash	115,009	96,983
Other Assets	290,530	240,369
Total	4,649,359	4,028,376
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	3,547,604	3,078,731
Total	3,547,604	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	4,243,820	3,691,024
Restricted Cash	106,351	95,966
Other Assets	280,646	240,208
Total	4,630,817	4,027,198
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	8,658	1,017
Other Assets	9,884	161
Total	$ 18,542	$ 1,178