Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	5,403,012	5,227,512
Total	5,403,012	5,227,512
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	993,810	992,200
Total	993,810	992,200
Secured Borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,409,202	4,235,312
Total	4,409,202	4,235,312
Secured Borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,409,202	4,235,312
Total	4,409,202	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	3,691,111	3,585,254
Total	3,691,111	3,585,254
Secured Borrowings | Variable-rate | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	718,091	650,058
Total	$ 718,091	$ 650,058