Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	6,036,527	5,227,512
Total	6,036,527	5,227,512
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	994,614	992,200
Total	994,614	992,200
Secured Borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	5,041,913	4,235,312
Total	5,041,913	4,235,312
Secured Borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	5,041,913	4,235,312
Total	5,041,913	4,235,312
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	4,204,755	3,585,254
Total	4,204,755	3,585,254
Secured Borrowings | Variable-rate | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	837,158	650,058
Total	$ 837,158	$ 650,058