Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,219,310	5,930,990
Total	5,219,310	5,930,990
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	147,104	210,741	$ 163,955
Other Assets	43,049	26,351
Total assets	27,498,773	29,221,899
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,231,926	4,994,852
Total	4,231,926	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,337,560	6,029,034
Restricted Cash	147,104	210,741
Other Assets	311,401	358,849
Total assets	5,796,065	6,598,624
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,231,926	4,994,852
Total	4,231,926	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,337,560	6,029,034
Restricted Cash	147,104	210,741
Other Assets	308,918	357,982
Total assets	5,793,582	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	2,483	867
Total assets	$ 2,483	$ 867