Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	6,147,473	6,440,345
Total	6,147,473	6,440,345
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	989,528	995,420
Total	989,528	995,420
Secured Borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	5,157,945	5,444,925
Total	5,157,945	5,444,925
Secured Borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	5,157,945	5,444,925
Total	5,157,945	5,444,925
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	4,389,633	4,617,743
Total	4,389,633	4,617,743
Secured Borrowings | Variable-rate | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	768,312	827,182
Total	$ 768,312	$ 827,182