Borrowings - Secured Financing (Details) - USD ($)	1 Months Ended
	Aug. 31, 2021	May 31, 2021	Mar. 31, 2022	Dec. 31, 2021
Securities Financing Transaction [Line Items]
Total Issued				$ 1,592,000,000
Total loans and accrued interest amount securitized at inception			$ 0	$ 1,656,263,000
2021-B
Securities Financing Transaction [Line Items]
Total Issued		$ 531,000,000
Basis spread on variable rate		0.77%
Weighted Average Life (in years)		4 years 3 months 3 days
2021-D
Securities Financing Transaction [Line Items]
Total Issued	$ 527,000,000
Basis spread on variable rate	0.69%
Weighted Average Life (in years)	4 years 2 months 19 days
2021-E
Securities Financing Transaction [Line Items]
Total Issued	$ 534,000,000
Basis spread on variable rate	0.69%
Weighted Average Life (in years)	4 years 1 month 24 days
Private Education Loan term securitizations
Securities Financing Transaction [Line Items]
Loans pledged as collateral			1,460,000,000
Principal amount of collateral			1,370,000,000
Capitalized interest			$ 87,000,000