Borrowings - Securitizations (Details) - USD ($)	1 Months Ended
	May 31, 2021	Aug. 31, 2020	Feb. 29, 2020	Jun. 30, 2021	Dec. 31, 2020
Securities Financing Transaction [Line Items]
Total Issued				$ 531,000,000	$ 1,343,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization				549,516,000	$ 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				1,240,000,000
Principal amount of collateral				1,150,000,000
Capitalized interest				85,000,000
SMB Private Education Loan Trust 2021-B
Securities Financing Transaction [Line Items]
Total Issued	$ 531,000,000
Basis spread on variable rate	0.77%
Weighted Average Life	4 years 3 months 3 days
Loans pledged as collateral				541,000,000
Principal amount of collateral				504,000,000
Capitalized interest				$ 37,000,000