Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2025	Dec. 31, 2024
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,199,417	$ 4,216,909
Long-term debt, weighted-average interest rate (percent)	3.06%	2.97%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 117,221	$ 128,234
Long-term debt, weighted-average interest rate (percent)	3.57%	3.30%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 2,900	$ 3,600
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 82,949	$ 93,495
Long-term debt, weighted-average interest rate (percent)	3.07%	2.66%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 34,631	$ 44,655
Long-term debt, weighted-average interest rate (percent)	3.50%	2.87%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 41,716	$ 42,208
Long-term debt, weighted-average interest rate (percent)	2.53%	2.21%
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	$ 6,602	$ 6,632
Long-term debt, weighted-average interest rate (percent)	4.03%	3.98%
Contractual maturity term	10 years	10 years
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 34,272	$ 34,739
Long-term debt, weighted-average interest rate (percent)	4.83%	5.07%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 32,437	$ 32,435
Long-term debt, weighted-average interest rate (percent)	4.47%	4.67%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 1,586	$ 2,055
Long-term debt, weighted-average interest rate (percent)	11.47%	10.99%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 249	$ 249
Long-term debt, weighted-average interest rate (percent)	8.95%	8.10%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,082,196	$ 4,088,675
Long-term debt, weighted-average interest rate (percent)	3.04%	2.96%