Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,443,715)	$ (1,435,920)	$ (1,350,058)	$ (1,339,772)
Private Education Loans, net	21,091,204	20,902,158
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	22,432,125	22,235,008
Deferred origination costs and unamortized premium/(discount)	102,794	103,070
Allowance for credit losses	(1,443,715)	(1,435,920)
Private Education Loans, net	21,091,204	20,902,158
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	17,463,708	17,093,382
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans, gross	$ 4,968,417	$ 5,141,626