Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,403,012	5,227,512
Total	5,403,012	5,227,512
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	142,230	149,669	$ 150,585
Other Assets	56,923	52,342
Total assets	28,768,844	29,169,468
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,409,202	4,235,312
Total	4,409,202	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,724,958	5,539,964
Restricted Cash	142,228	149,412
Other Assets	349,614	312,763
Total assets	6,216,800	6,002,139
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,409,202	4,235,312
Total	4,409,202	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,724,958	5,539,964
Restricted Cash	142,228	149,412
Other Assets	347,389	311,697
Total assets	6,214,575	6,001,073
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	2,225	1,066
Total assets	$ 2,225	$ 1,066