Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,497,173	$ 21,213,165	$ 20,805,777	$ 21,615,968
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,497,173	$ 20,649,663	$ 20,805,777	$ 21,032,541
Weighted Average Interest Rate	10.79%	10.96%	10.90%	10.80%
FFELP Loans(1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 0	$ 563,502	$ 0	$ 583,427
Weighted Average Interest Rate	0.00%	7.35%	0.00%	7.10%