Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	4,476,406	4,284,304
Total	4,476,406	4,284,304
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	153,552	122,789	$ 120,084
Other Assets	92,446	103,695
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	4,278,855	4,086,956
Total	4,278,855	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,251,117	5,030,837
Restricted Cash	143,307	113,431
Other Assets	357,439	326,727
Total	5,751,863	5,470,995
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	4,278,855	4,086,956
Total	4,278,855	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,251,117	5,030,837
Restricted Cash	143,307	113,431
Other Assets	356,496	326,570
Total	5,750,920	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	943	157
Total	$ 943	$ 157