Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,522,311	5,930,990
Total	5,522,311	5,930,990
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	177,917	210,741	$ 175,473
Other Assets	32,941	26,351
Total assets	29,139,088	29,221,899
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,534,129	4,994,852
Total	4,534,129	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,665,062	6,029,034
Restricted Cash	177,917	210,741
Other Assets	347,211	358,849
Total assets	6,190,190	6,598,624
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,534,129	4,994,852
Total	4,534,129	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,665,062	6,029,034
Restricted Cash	177,917	210,741
Other Assets	345,436	357,982
Total assets	6,188,415	6,597,757
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	1,775	867
Total assets	$ 1,775	$ 867