Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,515,532	5,235,114
Total	5,515,532	5,235,114
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	175,061	156,719	$ 177,917
Other Assets	46,787	34,073
Total assets	29,273,219	28,811,029
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,524,136	4,246,128
Total	4,524,136	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,776,779	5,433,602
Restricted Cash	174,757	156,719
Other Assets	359,987	287,159
Total assets	6,311,523	5,877,480
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,524,136	4,246,128
Total	4,524,136	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,776,779	5,433,602
Restricted Cash	174,757	156,719
Other Assets	358,220	286,093
Total assets	6,309,756	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	1,767	1,066
Total assets	$ 1,767	$ 1,066