Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	5,515,532	5,235,114
Total	5,515,532	5,235,114
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	991,396	988,986
Total	991,396	988,986
Secured Borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,524,136	4,246,128
Total	4,524,136	4,246,128
Secured Borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,524,136	4,246,128
Total	4,524,136	4,246,128
Secured Borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	0	0
Secured Borrowings | Fixed-rate | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,807,493	3,462,363
Total	3,807,493	3,462,363
Secured Borrowings | Variable-rate | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	716,643	783,765
Total	$ 716,643	$ 783,765