Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Secured borrowings:
Short-Term	$ 0	$ 0
Long-Term	6,440,345	5,227,512
Total	6,440,345	5,227,512
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	995,420	992,200
Total	995,420	992,200
Secured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	5,444,925	4,235,312
Total	5,444,925	4,235,312
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	5,444,925	4,235,312
Total	5,444,925	4,235,312
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	4,617,743	3,585,254
Total	4,617,743	3,585,254
Secured borrowings | Variable-rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	827,182	650,058
Total	$ 827,182	$ 650,058