Borrowings - Securitizations (Details) - USD ($)	1 Months Ended
	Aug. 31, 2020	Feb. 29, 2020	Mar. 31, 2021	Dec. 31, 2020
Securities Financing Transaction [Line Items]
Total Issued				$ 1,343,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization				$ 1,463,230,000
2020-A
Securities Financing Transaction [Line Items]
Total Issued		$ 636,000,000
Basis spread on variable rate		0.88%
Weighted Average Life		4 years 2 months 4 days
2020-B
Securities Financing Transaction [Line Items]
Total Issued	$ 707,000,000
Basis spread on variable rate	1.30%
Weighted Average Life	4 years 1 month 20 days
Private Education Loan securitization
Securities Financing Transaction [Line Items]
Loans pledged as collateral			$ 1,290,000,000
Principal amount of collateral			1,200,000,000
Capitalized interest			$ 86,000,000