Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,134,424	$ 3,923,563
Long-term debt, weighted-average interest rate (percent)	1.74%	1.85%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 226,798	$ 277,399
Long-term debt, weighted-average interest rate (percent)	1.48%	1.34%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 1,300	$ 392
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 138,666	$ 161,916
Long-term debt, weighted-average interest rate (percent)	1.82%	1.69%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 93,233	$ 106,691
Long-term debt, weighted-average interest rate (percent)	2.16%	2.03%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 38,385	$ 48,524
Long-term debt, weighted-average interest rate (percent)	0.62%	0.63%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,048	$ 6,701
Long-term debt, weighted-average interest rate (percent)	3.80%	3.90%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 88,132	$ 115,483
Long-term debt, weighted-average interest rate (percent)	0.94%	0.86%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 74,078	$ 100,089
Long-term debt, weighted-average interest rate (percent)	0.33%	0.35%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 13,666	$ 14,978
Long-term debt, weighted-average interest rate (percent)	4.11%	4.16%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 388	$ 416
Long-term debt, weighted-average interest rate (percent)	8.16%	7.75%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,907,626	$ 3,646,164
Long-term debt, weighted-average interest rate (percent)	1.75%	1.88%