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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Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ (1,435,920)	$ (1,339,772)	$ (1,357,075)	$ (1,165,335)
Loans held for investment, net	20,902,158	20,306,357
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	22,235,008	21,025,844
Deferred origination costs and unamortized premium/ (discount)	103,070	81,554
Allowance for loan losses	(1,435,920)	(1,335,105)
Loans held for investment, net	20,902,158	19,772,293
Private Education Loans \| Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	17,093,382	13,985,791
Private Education Loans \| Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	5,141,626	7,040,053
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	0	537,401
Deferred origination costs and unamortized premium/ (discount)	0	1,330
Allowance for loan losses	0	(4,667)
Loans held for investment, net	$ 0	$ 534,064