Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,204,825	$ 4,155,396
Long-term debt, weighted-average interest rate (percent)	2.29%	1.88%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 126,787	$ 198,097
Long-term debt, weighted-average interest rate (percent)	2.25%	1.55%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 5,100	$ 1,600
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 119,578	$ 134,975
Long-term debt, weighted-average interest rate (percent)	1.96%	1.78%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 74,191	$ 89,618
Long-term debt, weighted-average interest rate (percent)	2.40%	2.13%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 38,638	$ 38,312
Long-term debt, weighted-average interest rate (percent)	0.70%	0.60%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,749	$ 7,045
Long-term debt, weighted-average interest rate (percent)	4.06%	3.73%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,209	$ 63,122
Long-term debt, weighted-average interest rate (percent)	7.29%	1.05%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 0	$ 51,583
Long-term debt, weighted-average interest rate (percent)	0.00%	0.32%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,955	$ 11,166
Long-term debt, weighted-average interest rate (percent)	7.22%	4.30%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 254	$ 373
Long-term debt, weighted-average interest rate (percent)	9.26%	7.17%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,078,038	$ 3,957,299
Long-term debt, weighted-average interest rate (percent)	2.29%	1.89%