Borrowings - Variable Interest Entity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	5,930,990	5,189,217
Total	5,930,990	5,189,217
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	210,741	154,417	$ 156,883
Other assets	26,351	19,533
Total	29,221,899	30,770,423
Secured Borrowing Facility
Debt Outstanding
Short-Term	0	0
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,994,852	4,496,338
Total	4,994,852	4,496,338
Carrying Amount of Assets Securing Debt Outstanding
Loans	6,029,034	5,661,123
Restricted Cash	210,741	154,417
Other assets	358,849	357,403
Total	6,598,624	6,172,943
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	4,994,852	4,496,338
Total	4,994,852	4,496,338
Carrying Amount of Assets Securing Debt Outstanding
Loans	6,029,034	5,661,123
Restricted Cash	210,741	154,417
Other assets	357,982	356,967
Total	6,597,757	6,172,507
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	867	436
Total	$ 867	$ 436