Loans Held for Investment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	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	29.00%	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	$ 143
Proceeds from sale of loans receivable	2,100
Net proceeds from sales of loans held for investment, principal	1,950
Net proceeds from sales of loans held for investment, capitalized interest	$ 151