Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,235,114	5,930,990
Total	5,235,114	5,930,990
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	156,719	210,741	$ 154,417
Other assets	34,073	26,351
Total	28,811,029	29,221,899
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,246,128	4,994,852
Total	4,246,128	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,433,602	6,029,034
Restricted Cash	156,719	210,741
Other assets	287,159	358,849
Total	5,877,480	6,598,624
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	4,246,128	4,994,852
Total	4,246,128	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,433,602	6,029,034
Restricted Cash	156,719	210,741
Other assets	286,093	357,982
Total	5,876,414	6,597,757
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	867
Total	$ 1,066	$ 867