Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Aug. 31, 2023	Mar. 31, 2023	Aug. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Total Issued				$ 1,147,000,000	$ 575,000,000
Total loans and accrued interest amount securitized at inception				1,292,507,000	674,387,000
2022-C
Debt Instrument [Line Items]
Total Issued			$ 575,000,000
Weighted Average Life (in years)			4 years 8 months 8 days
Principal amount of collateral					513,000,000
Capitalized interest					$ 30,000,000
2022-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Weighted Average Cost of Funds			1.76%
2023-A
Debt Instrument [Line Items]
Total Issued		$ 579,000,000
Weighted Average Life (in years)		5 years 21 days
Principal amount of collateral				551,000,000
Capitalized interest				40,000,000
2023-A | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Weighted Average Cost of Funds		1.53%
2023-C
Debt Instrument [Line Items]
Total Issued	$ 568,000,000
Weighted Average Life (in years)	4 years 11 months 4 days
Principal amount of collateral				579,000,000
Capitalized interest				41,000,000
2023-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Weighted Average Cost of Funds	1.69%
Private Education Loans Securitized in 2021
Debt Instrument [Line Items]
Loans				543,000,000
Principal amount of collateral				513,000,000
Capitalized interest				30,000,000
Private Education Loans Securitized in 2022
Debt Instrument [Line Items]
Loans				1,210,000,000
Principal amount of collateral				1,130,000,000
Capitalized interest				$ 81,000,000