Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,350,058)	$ (1,339,772)	$ (1,479,306)	$ (1,357,075)
Private Education Loans, net	20,200,789	20,306,357
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	20,952,346	21,025,844
Deferred origination costs and unamortized premium/(discount)	80,868	81,554
Allowance for credit losses	(1,345,431)	(1,335,105)
Private Education Loans, net	19,687,783	19,772,293
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	14,788,341	13,985,791
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	6,164,005	7,040,053
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	516,363	537,401
Deferred origination costs and unamortized premium/(discount)	1,270	1,330
Allowance for credit losses	(4,627)	(4,667)
Private Education Loans, net	$ 513,006	$ 534,064