Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Debt Outstanding
Short Term	$ 297,800	$ 0
Long Term	4,601,888	4,284,304
Total	4,899,688	4,284,304
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	142,846	122,789	$ 115,658
Other Assets	40,324	48,141
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	297,800	0
Long Term	4,403,932	4,086,956
Total	4,701,732	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,786,175	5,030,837
Restricted Cash	135,945	113,431
Other Assets	427,733	326,727
Total	6,349,853	5,470,995
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	4,403,932	4,086,956
Total	4,403,932	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,446,885	5,030,837
Restricted Cash	127,043	113,431
Other Assets	400,574	326,570
Total	5,974,502	5,470,838
Variable Interest Entity, Primary Beneficiary | Secured Borrowing Facility
Debt Outstanding
Short Term	297,800	0
Long Term	0	0
Total	297,800	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	339,290	0
Restricted Cash	8,902	0
Other Assets	27,159	157
Total	$ 375,351	$ 157