Short-Term and Long-Term Debt - Short-Term Debt (Details) - Fannie Mae - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 14,285	$ 17,314
Short-term debt, weighted-average interest rate (percent)	5.18%	5.13%