Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,613,919	$ 21,238,931	$ 21,701,229
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,039,701	$ 20,576,737	$ 20,968,061
Weighted Average Interest Rate	10.86%	9.14%	8.25%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 574,218	$ 662,194	$ 718,186
Weighted Average Interest Rate	7.19%	4.62%	3.43%
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 0	$ 0	$ 14,982
Weighted Average Interest Rate	0.00%	0.00%	4.67%