Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,552,497	5,930,990
Total	5,552,497	5,930,990
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	187,960	210,741	$ 164,708
Other Assets	24,855	26,351
Total assets	29,051,829	29,221,899
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,565,906	4,994,852
Total	4,565,906	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,650,067	6,029,034
Restricted Cash	187,960	210,741
Other Assets	333,623	358,849
Total assets	6,171,650	6,598,624
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	4,565,906	4,994,852
Total	4,565,906	4,994,852
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,650,067	6,029,034
Restricted Cash	187,960	210,741
Other Assets	333,325	357,982
Total assets	6,171,352	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	298	867
Total assets	$ 298	$ 867