Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	6,036,527	5,227,512
Total	6,036,527	5,227,512
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	170,984	149,669	$ 175,061
Other Assets	54,914	52,342
Total assets	30,012,767	29,169,468
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	5,041,913	4,235,312
Total	5,041,913	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	6,344,419	5,539,964
Restricted Cash	170,982	149,412
Other Assets	421,581	312,763
Total assets	6,936,982	6,002,139
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	5,041,913	4,235,312
Total	5,041,913	4,235,312
Carrying Amount of Assets Securing Debt Outstanding
Loans	6,344,419	5,539,964
Restricted Cash	170,982	149,412
Other Assets	419,945	311,697
Total assets	6,935,346	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	1,636	1,066
Total assets	$ 1,636	$ 1,066