Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt Outstanding
Short-Term	$ 0	$ 289,230
Long-Term	4,947,647	4,354,037
Total	4,947,647	4,643,267
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	162,448	156,883	$ 142,846
Other Assets	23,921	29,398
Total assets	30,642,421	32,686,479
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	289,230
Long-Term	4,748,879	4,155,878
Total	4,748,879	4,445,108
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,865,487	5,586,652
Restricted Cash	162,448	154,563
Other Assets	427,652	357,005
Total assets	6,455,587	6,098,220
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short-Term	0	0
Long-Term	4,748,879	4,155,878
Total	4,748,879	4,155,878
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,865,487	5,246,986
Restricted Cash	162,448	145,760
Other Assets	426,404	333,173
Total assets	6,454,339	5,725,919
Variable Interest Entity, Primary Beneficiary | Secured Borrowing Facility
Debt Outstanding
Short-Term	0	289,230
Long-Term	0	0
Total	0	289,230
Carrying Amount of Assets Securing Debt Outstanding
Loans	0	339,666
Restricted Cash	0	8,803
Other Assets	1,248	23,832
Total assets	$ 1,248	$ 372,301