Loans Held for Investment - Loans Summary (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,449,614)	$ (1,430,318)
Private Education Loans, net	20,626,170	20,332,124
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	21,966,017	21,660,434
Deferred origination costs and unamortized premium/(discount)	109,767	102,008
Allowance for credit losses	(1,449,614)	(1,430,318)
Private Education Loans, net	20,626,170	20,332,124
Private Education Loans \| Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	17,192,505	16,952,620
Private Education Loans \| Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	$ 4,773,512	$ 4,707,814

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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