Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Outstanding
Short Term	$ 300,000	$ 0
Long Term	2,738,662	2,167,979
Total	3,038,662	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	66,625	53,717
Other Assets	74,258	52,153
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	300,000	0
Long Term	2,542,325	2,167,979
Total	2,842,325	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,437,211	2,562,156
Restricted Cash	63,226	44,617
Other Assets	254,869	160,783
Total	3,755,306	2,767,556
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	2,542,325	2,167,979
Total	2,542,325	2,167,979
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,067,431	2,562,156
Restricted Cash	57,687	44,617
Other Assets	229,797	160,783
Total	3,354,915	2,767,556
Variable Interest Entity, Primary Beneficiary | ABCP Facility
Debt Outstanding
Short Term	300,000	0
Long Term	0	0
Total	300,000	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	369,780	0
Restricted Cash	5,539	0
Other Assets	25,072	0
Total	$ 400,391	$ 0