Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	2,872,231	2,167,979
Total	2,872,231	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	62,466	53,717
Other Assets	45,841	52,153
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	2,675,491	2,167,979
Total	2,675,491	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,172,113	2,562,156
Restricted Cash	57,370	44,617
Other Assets	224,768	160,783
Total	3,454,251	2,767,556
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	2,675,491	2,167,979
Total	2,675,491	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,172,113	2,562,156
Restricted Cash	57,370	44,617
Other Assets	224,768	160,783
Total	3,454,251	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