Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Aug. 31, 2022	Nov. 30, 2021	Aug. 31, 2021	May 31, 2021	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Total Issued					$ 575,000,000	$ 1,592,000,000
Total loans and accrued interest amount securitized at inception					674,387,000	$ 1,656,263,000
2021-B
Debt Instrument [Line Items]
Total Issued				$ 531,000,000
Weighted Average Life (in years)				4 years 3 months 3 days
Principal amount of collateral					389,000,000
Capitalized interest					21,000,000
2021-B | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate				0.77%
2021-D
Debt Instrument [Line Items]
Total Issued			$ 527,000,000
Weighted Average Life (in years)			4 years 2 months 19 days
Loans					425,000,000
Principal amount of collateral					403,000,000
Capitalized interest					22,000,000
2021-D | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			0.69%
2021-E
Debt Instrument [Line Items]
Total Issued		$ 534,000,000
Weighted Average Life (in years)		4 years 1 month 24 days
Loans					439,000,000
Principal amount of collateral					416,000,000
Capitalized interest					23,000,000
2021-E | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate		0.69%
SMB Private Education Loan Trust 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					597,000,000
Capitalized interest					38,000,000
SMB Private Education Loan Trust 2022-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.76%
Private Education Loans Securitized in 2021
Debt Instrument [Line Items]
Loans					1,270,000,000
Principal amount of collateral					1,210,000,000
Capitalized interest					66,000,000
Private Education Loans Securitized in 2022
Debt Instrument [Line Items]
Loans					635,000,000
Principal amount of collateral					597,000,000
Capitalized interest					$ 38,000,000