Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Debt Outstanding
Short-Term	$ 0	$ 289,230
Long-Term	4,708,036	4,354,037
Total	4,708,036	4,643,267
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	189,439	156,883	$ 153,552
Other Assets	41,812	29,398
Total assets	31,760,895	32,686,479
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	289,230
Long-Term	4,509,674	4,155,878
Total	4,509,674	4,445,108
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,594,488	5,586,652
Restricted Cash	181,862	154,563
Other Assets	372,630	357,005
Total assets	6,148,980	6,098,220
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short-Term	0	0
Long-Term	4,509,674	4,155,878
Total	4,509,674	4,155,878
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,594,488	5,246,986
Restricted Cash	172,546	145,760
Other Assets	370,795	333,173
Total assets	6,137,829	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	9,316	8,803
Other Assets	1,835	23,832
Total assets	$ 11,151	$ 372,301