Short-Term and Long-Term Debt - Short-Term Debt (Details) - Fannie Mae - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Short-Term Debt [Line Items]
Short-term debt, outstanding	$ 4,045	$ 2,795
Short-term debt, weighted-average interest rate (percent)	0.15%	0.03%