Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,200,956	$ 4,205,404
Long-term debt, weighted-average interest rate (percent)	2.84%	2.57%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 106,535	$ 106,751
Long-term debt, weighted-average interest rate (percent)	2.88%	2.63%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 4,100	$ 4,000
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 100,961	$ 103,731
Long-term debt, weighted-average interest rate (percent)	2.61%	2.40%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 52,653	$ 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	$ 41,567	$ 42,217
Long-term debt, weighted-average interest rate (percent)	1.95%	1.58%
Fannie Mae | Senior fixed medium-term notes | Minimum
Long-Term Debt [Line Items]
Contractual maturity term	1 year
Fannie Mae | Senior fixed medium-term notes | Maximum
Long-Term Debt [Line Items]
Contractual maturity term	10 years
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,741	$ 6,787
Long-term debt, weighted-average interest rate (percent)	4.13%	3.98%
Contractual maturity term	10 years
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,574	$ 3,020
Long-term debt, weighted-average interest rate (percent)	8.05%	10.92%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,999	$ 0
Long-term debt, weighted-average interest rate (percent)	5.45%	0.00%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,305	$ 2,752
Long-term debt, weighted-average interest rate (percent)	11.36%	11.12%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 270	$ 268
Long-term debt, weighted-average interest rate (percent)	8.81%	8.79%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,094,421	$ 4,098,653
Long-term debt, weighted-average interest rate (percent)	2.84%	2.56%