Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,202,289	$ 4,205,404
Long-term debt, weighted-average interest rate (percent)	2.81%	2.57%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 104,116	$ 106,751
Long-term debt, weighted-average interest rate (percent)	2.76%	2.63%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 4,200	$ 4,000
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 101,250	$ 103,731
Long-term debt, weighted-average interest rate (percent)	2.54%	2.40%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 52,544	$ 54,727
Long-term debt, weighted-average interest rate (percent)	2.90%	2.79%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 41,950	$ 42,217
Long-term debt, weighted-average interest rate (percent)	1.80%	1.58%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,756	$ 6,787
Long-term debt, weighted-average interest rate (percent)	4.13%	3.98%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,866	$ 3,020
Long-term debt, weighted-average interest rate (percent)	10.90%	10.92%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,593	$ 2,752
Long-term debt, weighted-average interest rate (percent)	11.10%	11.12%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 273	$ 268
Long-term debt, weighted-average interest rate (percent)	8.79%	8.79%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,098,173	$ 4,098,653
Long-term debt, weighted-average interest rate (percent)	2.82%	2.56%