Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,213,698	5,235,114
Total	5,213,698	5,235,114
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	150,585	156,719	$ 147,104
Other Assets	53,075	34,073
Total assets	27,708,403	28,811,029
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,223,106	4,246,128
Total	4,223,106	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,461,684	5,433,602
Restricted Cash	150,585	156,719
Other Assets	313,691	287,159
Total assets	5,925,960	5,877,480
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,223,106	4,246,128
Total	4,223,106	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,461,684	5,433,602
Restricted Cash	150,585	156,719
Other Assets	311,216	286,093
Total assets	5,923,485	5,876,414
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	2,475	1,066
Total assets	$ 2,475	$ 1,066