Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,513,976	5,235,114
Total	5,513,976	5,235,114
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	181,764	156,719	$ 187,960
Other Assets	43,548	34,073
Total assets	29,453,901	28,811,029
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,524,188	4,246,128
Total	4,524,188	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,728,010	5,433,602
Restricted Cash	181,764	156,719
Other Assets	323,075	287,159
Total assets	6,232,849	5,877,480
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,524,188	4,246,128
Total	4,524,188	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,728,010	5,433,602
Restricted Cash	181,764	156,719
Other Assets	322,715	286,093
Total assets	6,232,489	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	360	1,066
Total assets	$ 360	$ 1,066