Loans Held for Investment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	35.00%		35.00%		45.00%
Tier 1 of government guarantee (at least)	97.00%		97.00%			97.00%
Tier 2 of government guarantee							98.00%
Tier 3 of government guarantee								100.00%
Gain on sales of loans, net	$ (5)	$ 74,978	$ 124,740	$ 324,856
Estimated weighted average life of student loans			5 years		5 years
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on sales of loans, net			$ 128,000	325,000
Proceeds from sale of loans receivable			2,100,000	3,290,000
Net proceeds from sales of loans held for investment, principal			1,960,000	3,080,000
Net proceeds from sales of loans held for investment, capitalized interest			$ 144,000	$ 213,000