Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	4,862,763	4,284,304
Total	4,862,763	4,284,304
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	141,441	122,789	$ 114,659
Other Assets	116,207	103,695
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	4,665,010	4,086,956
Total	4,665,010	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,691,009	5,030,837
Restricted Cash	134,607	113,431
Other Assets	404,411	326,727
Total	6,230,027	5,470,995
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	4,665,010	4,086,956
Total	4,665,010	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,691,009	5,030,837
Restricted Cash	134,607	113,431
Other Assets	403,697	326,570
Total	6,229,313	5,470,838
Variable Interest Entity, Primary Beneficiary | Secured Borrowing 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	714	157
Total	$ 714	$ 157