Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Aug. 31, 2022	Nov. 30, 2021	Aug. 31, 2021	May 31, 2021	Sep. 30, 2022	Aug. 30, 2022	Dec. 31, 2021
Securities Financing Transaction [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
Securities Financing Transaction [Line Items]
Total Issued				$ 531,000,000
Weighted Average Life (in years)				4 years 3 months 3 days
2021-B | London Interbank Offered Rate (LIBOR)
Securities Financing Transaction [Line Items]
Basis spread on variable rate				0.77%
2021-D
Securities Financing Transaction [Line Items]
Total Issued			$ 527,000,000
Weighted Average Life (in years)			4 years 2 months 19 days
2021-D | London Interbank Offered Rate (LIBOR)
Securities Financing Transaction [Line Items]
Basis spread on variable rate			0.69%
2021-E
Securities Financing Transaction [Line Items]
Total Issued		$ 534,000,000
Weighted Average Life (in years)		4 years 1 month 24 days
2021-E | London Interbank Offered Rate (LIBOR)
Securities Financing Transaction [Line Items]
Basis spread on variable rate		0.69%
2022-C
Securities Financing Transaction [Line Items]
Total Issued						$ 575,000,000
Weighted Average Life (in years)	4 years 8 months 8 days
Principal amount of collateral					613,000,000
Capitalized interest					45,000,000
2022-C | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Securities Financing Transaction [Line Items]
Basis spread on variable rate						1.76%
Private Education Loans Securitized in 2021
Securities Financing Transaction [Line Items]
Loans					1,330,000,000
Principal amount of collateral					1,250,000,000
Capitalized interest					84,000,000
Private Education Loans Securitized in 2022
Securities Financing Transaction [Line Items]
Loans					658,000,000
Principal amount of collateral					613,000,000
Capitalized interest					$ 45,000,000