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Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,469,509)	$ (1,435,920)	$ (1,269,652)	$ (1,350,058)	$ (1,339,772)
Private Education Loans, net	21,160,332	20,902,158
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	22,525,817	22,235,008
Deferred origination costs and unamortized premium/(discount)	104,024	103,070
Allowance for credit losses	(1,469,509)	(1,435,920)
Private Education Loans, net	21,160,332	20,902,158
Private Education Loans \| Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	17,646,982	17,093,382
Private Education Loans \| Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	$ 4,878,835	$ 5,141,626