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Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,526,104)	$ (1,435,920)	$ (1,413,621)	$ (1,269,652)	$ (1,339,772)
Private Education Loans, net	21,615,067	20,902,158
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	23,029,999	22,235,008
Deferred origination costs and unamortized premium/(discount)	111,172	103,070
Allowance for credit losses	(1,526,104)	(1,435,920)
Private Education Loans, net	21,615,067	20,902,158
Private Education Loans \| Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	18,112,352	17,093,382
Private Education Loans \| Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	$ 4,917,647	$ 5,141,626