Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,206,359	$ 4,205,404
Long-term debt, weighted-average interest rate (percent)	2.91%	2.57%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 110,296	$ 106,751
Long-term debt, weighted-average interest rate (percent)	3.11%	2.63%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 3,400	$ 4,000
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 92,876	$ 103,731
Long-term debt, weighted-average interest rate (percent)	2.57%	2.40%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 46,825	$ 54,727
Long-term debt, weighted-average interest rate (percent)	2.89%	2.79%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 39,231	$ 42,217
Long-term debt, weighted-average interest rate (percent)	1.90%	1.58%
Fannie Mae | Senior fixed medium-term notes | Minimum
Long-Term Debt [Line Items]
Contractual maturity term	1 year	1 year
Fannie Mae | Senior fixed medium-term notes | Maximum
Long-Term Debt [Line Items]
Contractual maturity term	10 years	10 years
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,820	$ 6,787
Long-term debt, weighted-average interest rate (percent)	4.08%	3.98%
Contractual maturity term	10 years
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 17,420	$ 3,020
Long-term debt, weighted-average interest rate (percent)	6.08%	10.92%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 14,997	$ 0
Long-term debt, weighted-average interest rate (percent)	5.25%	0.00%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,139	$ 2,752
Long-term debt, weighted-average interest rate (percent)	11.56%	11.12%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 284	$ 268
Long-term debt, weighted-average interest rate (percent)	8.81%	8.79%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,096,063	$ 4,098,653
Long-term debt, weighted-average interest rate (percent)	2.90%	2.56%