Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Mar. 31, 2023	Aug. 31, 2022	Jun. 30, 2023	Dec. 31, 2022
Securities Financing Transaction [Line Items]
Total Issued			$ 579,000,000	$ 575,000,000
Total loans and accrued interest amount securitized at inception			644,573,000	$ 674,387,000
2022-C
Securities Financing Transaction [Line Items]
Total Issued		$ 575,000,000
Weighted Average Life (in years)		4 years 8 months 8 days
2022-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Securities Financing Transaction [Line Items]
Basis spread on variable rate		1.76%
2023-A
Securities Financing Transaction [Line Items]
Total Issued	$ 579,000,000
Weighted Average Life (in years)	5 years 21 days
Principal amount of collateral			581,000,000
Capitalized interest			44,000,000
2023-A | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Securities Financing Transaction [Line Items]
Basis spread on variable rate	1.53%
Private Education Loans Securitized in 2022
Securities Financing Transaction [Line Items]
Loans			588,000,000
Principal amount of collateral			550,000,000
Capitalized interest			38,000,000
Private Education Loans Securitized In 2023
Securities Financing Transaction [Line Items]
Loans			625,000,000
Principal amount of collateral			581,000,000
Capitalized interest			$ 44,000,000