Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,269,652)	$ (1,350,058)	$ (1,339,772)	$ (1,364,716)	$ (1,479,306)	$ (1,357,075)
Private Education Loans, net	18,915,333		20,306,357
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	19,619,531		21,025,844
Deferred origination costs and unamortized premium/(discount)	78,661		81,554
Allowance for credit losses	(1,265,592)		(1,335,105)
Private Education Loans, net	18,432,600		19,772,293
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	14,124,662		13,985,791
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	5,494,869		7,040,053
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	485,608		537,401
Deferred origination costs and unamortized premium/(discount)	1,185		1,330
Allowance for credit losses	(4,060)		(4,667)
Private Education Loans, net	$ 482,733		$ 534,064