Borrowings - Variable Interest Entity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	4,284,304	3,275,270
Total	4,284,304	3,275,270
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	122,789	101,836	$ 53,717
Other assets	103,695	84,853
Secured Borrowing Facility
Debt Outstanding
Short-Term	0	0
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,086,956	3,078,731
Total	4,086,956	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,030,837	3,691,024
Restricted Cash	113,431	96,983
Other assets	326,727	240,369
Total	5,470,995	4,028,376
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	4,086,956	3,078,731
Total	4,086,956	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,030,837	3,691,024
Restricted Cash	113,431	95,966
Other assets	326,570	240,208
Total	5,470,838	4,027,198
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	1,017
Other assets	157	161
Total	$ 157	$ 1,178