Loans Held for Investment - Student Loan Portfolio by Program (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ (1,416,048)	$ (1,364,716)	$ (1,357,075)	$ (1,194,238)	$ (1,081,066)	$ (1,165,335)
Private Education Loans, net	20,899,181		19,626,868
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	21,680,867		20,303,688
Deferred origination costs and unamortized premium/(discount)	78,673		69,656
Allowance for credit losses	(1,411,232)		(1,353,631)
Private Education Loans, net	20,348,308		19,019,713
Private Education Loans | Fixed-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	14,000,110		11,108,079
Private Education Loans | Variable-rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	7,680,757		9,195,609
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan type	554,309		609,050
Deferred origination costs and unamortized premium/(discount)	1,380		1,549
Allowance for credit losses	(4,816)		(3,444)
Private Education Loans, net	$ 550,873		$ 607,155