Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Outstanding
Short-Term	$ 199,583	$ 0
Long-Term	5,219,748	5,189,217
Total	5,419,331	5,189,217
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	175,473	154,417	$ 162,448
Other Assets	32,506	19,533
Total assets	28,791,139	30,770,423
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	4,725,163	4,496,338
Total	4,725,163	4,496,338
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,788,954	5,661,123
Restricted Cash	175,472	154,417
Other Assets	409,070	357,403
Total assets	6,373,496	6,172,943
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short-Term	0	0
Long-Term	4,725,163	4,496,338
Total	4,725,163	4,496,338
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,788,954	5,661,123
Restricted Cash	175,472	154,417
Other Assets	407,634	356,967
Total assets	6,372,060	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	1,436	436
Total assets	$ 1,436	$ 436