Loans Held for Investment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Receivables [Abstract]
Percent of private loans indexed to LIBOR	47.00%		47.00%		52.00%
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 from sale	$ 74,978	$ (10)	$ 324,856	$ 402,780
Proceeds from sale of loans			3,290,000	3,190,000
Net proceeds from sales of loans held for investment, principal			3,080,000	2,990,000
Net proceeds from sales of loans held for investment, capitalized interest			$ 213,000	$ 195,000
Estimated weighted average life of student loans			5 years		4 years 8 months 12 days