Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,183,413	$ 4,216,909
Long-term debt, weighted-average interest rate (percent)	3.12%	2.97%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 106,468	$ 128,234
Long-term debt, weighted-average interest rate (percent)	3.87%	3.30%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 2,700	$ 3,600
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 72,264	$ 93,495
Long-term debt, weighted-average interest rate (percent)	3.44%	2.66%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 30,304	$ 44,655
Long-term debt, weighted-average interest rate (percent)	3.89%	2.87%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 35,351	$ 42,208
Long-term debt, weighted-average interest rate (percent)	2.92%	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	$ 6,609	$ 6,632
Long-term debt, weighted-average interest rate (percent)	3.97%	3.98%
Contractual maturity term	10 years	10 years
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 34,204	$ 34,739
Long-term debt, weighted-average interest rate (percent)	4.81%	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)	4.46%	4.67%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 1,509	$ 2,055
Long-term debt, weighted-average interest rate (percent)	11.54%	10.99%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 258	$ 249
Long-term debt, weighted-average interest rate (percent)	8.95%	8.10%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,076,945	$ 4,088,675
Long-term debt, weighted-average interest rate (percent)	3.10%	2.96%