Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2025	Sep. 30, 2026	Dec. 31, 2025
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 4,138,129	$ 4,155,891
Long-term debt, weighted-average interest rate (percent)		3.27%	3.16%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 137,611	$ 102,751
Long-term debt, weighted-average interest rate (percent)		4.06%	3.89%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments		$ 2,800	$ 2,600
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 85,357	$ 64,688
Long-term debt, weighted-average interest rate (percent)		4.12%	3.70%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 19,470	$ 26,649
Long-term debt, weighted-average interest rate (percent)		5.06%	4.16%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 59,506	$ 31,430
Long-term debt, weighted-average interest rate (percent)		3.80%	3.25%
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		$ 6,381	$ 6,609
Long-term debt, weighted-average interest rate (percent)		3.95%	3.87%
Contractual maturity term	10 years	10 years
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 52,254	$ 38,063
Long-term debt, weighted-average interest rate (percent)		3.95%	4.23%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 51,435	$ 36,437
Long-term debt, weighted-average interest rate (percent)		3.86%	3.93%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 819	$ 1,626
Long-term debt, weighted-average interest rate (percent)		9.23%	10.99%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding		$ 4,000,518	$ 4,053,140
Long-term debt, weighted-average interest rate (percent)		3.24%	3.14%