Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Short-Term	$ 199,379	$ 0
Long-Term	4,989,060	5,189,217
Total	5,188,439	5,189,217
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	199,379	0
Long-Term	494,255	692,879
Total	693,634	692,879
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,494,805	4,496,338
Total	4,494,805	4,496,338
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,494,805	4,496,338
Total	4,494,805	4,496,338
Secured borrowings | Private Education Loan securitization | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,477,121	3,261,233
Total	3,477,121	3,261,233
Secured borrowings | Private Education Loan securitization | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	1,017,684	1,235,105
Total	1,017,684	1,235,105
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0