Loans Held for Investment - Narrative (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	24.00%		33.00%
Tier 1 of government guarantee (at least)	97.00%			97.00%
Tier 2 of government guarantee					98.00%
Tier 3 of government guarantee						100.00%
Estimated weighted average life of student loans	5 years 4 months 24 days		5 years
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on sales of loans, net	$ 255	$ 128
Proceeds from sale of loans receivable	3,690	2,100
Net proceeds from sales of loans held for investment, principal	3,420	1,960
Net proceeds from sales of loans held for investment, capitalized interest	$ 274	$ 144