Borrowings - Variable Interest Entity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Outstanding
Short-Term	$ 289,230	$ 0
Long-Term	4,354,037	4,284,304
Total	4,643,267	4,284,304
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	156,883	122,789	$ 101,836
Other assets	29,398	48,141
Secured Borrowing Facility
Debt Outstanding
Short-Term	289,230	0
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	289,230	0
Long-Term	4,155,878	4,086,956
Total	4,445,108	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,586,652	5,030,837
Restricted Cash	154,563	113,431
Other assets	357,005	326,727
Total	6,098,220	5,470,995
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	4,155,878	4,086,956
Total	4,155,878	4,086,956
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,246,986	5,030,837
Restricted Cash	145,760	113,431
Other assets	333,173	326,570
Total	5,725,919	5,470,838
Variable Interest Entity, Primary Beneficiary | Secured Borrowing Facility
Debt Outstanding
Short-Term	289,230	0
Long-Term	0	0
Total	289,230	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	339,666	0
Restricted Cash	8,803	0
Other assets	23,832	157
Total	$ 372,301	$ 157