Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Aug. 31, 2023	Mar. 31, 2023	Mar. 31, 2024	Dec. 31, 2023
Securities Financing Transaction [Line Items]
Total Issued				$ 1,147,000,000
Total loans and accrued interest amount securitized at inception				$ 1,292,507,000
2023-A
Securities Financing Transaction [Line Items]
Total Issued		$ 579,000,000
Weighted Average Life (in years)		5 years 21 days
2023-A | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Securities Financing Transaction [Line Items]
Weighted Average Cost of Funds (as a percent)		1.53%
2023-C
Securities Financing Transaction [Line Items]
Total Issued	$ 568,000,000
Weighted Average Life (in years)	4 years 11 months 4 days
2023-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Securities Financing Transaction [Line Items]
Weighted Average Cost of Funds (as a percent)	1.69%
Private Education Loans Securitized In 2023
Securities Financing Transaction [Line Items]
Loans			$ 1,170,000,000
Principal amount of collateral			1,090,000,000.00
Capitalized interest			$ 84,000,000