Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,227,512	5,235,114
Total	5,227,512	5,235,114
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	149,669	156,719	$ 210,741
Other assets	52,342	34,073
Total	29,169,468	28,811,029
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,235,312	4,246,128
Total	4,235,312	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,539,964	5,433,602
Restricted Cash	149,412	156,719
Other assets	312,763	287,159
Total	6,002,139	5,877,480
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	4,235,312	4,246,128
Total	4,235,312	4,246,128
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,539,964	5,433,602
Restricted Cash	149,412	156,719
Other assets	311,697	286,093
Total	6,001,073	5,876,414
Variable Interest Entity, Primary Beneficiary | Fixed-rate borrowings
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	1,066	1,066
Total	$ 1,066	$ 1,066