Short-Term and Long-Term Debt - Short-Term Debt (Details) - Fannie Mae - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Short-Term Debt [Line Items]
Short-term debt of Fannie Mae, outstanding	$ 17,314	$ 10,204
Short-term debt of Fannie Mae, weighted-average interest rate	5.13%	3.93%