Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Secured borrowings:
Short-Term	$ 0	$ 0
Long-Term	5,227,512	5,235,114
Total	5,227,512	5,235,114
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	992,200	988,986
Total	992,200	988,986
Secured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	4,235,312	4,246,128
Total	4,235,312	4,246,128
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	4,235,312	4,246,128
Total	4,235,312	4,246,128
Secured borrowings | Secured Borrowing Facility
Secured borrowings:
Short-Term	0	0
Long-Term	0	0
Total	0	0
Secured borrowings | Fixed-rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	3,585,254	3,462,363
Total	3,585,254	3,462,363
Secured borrowings | Variable-rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	650,058	783,765
Total	$ 650,058	$ 783,765