Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,093,995	$ 3,923,563
Long-term debt, weighted-average interest rate (percent)	1.83%	1.85%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 249,296	$ 277,399
Long-term debt, weighted-average interest rate (percent)	1.39%	1.34%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 1,100	$ 392
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 147,484	$ 161,916
Long-term debt, weighted-average interest rate (percent)	1.77%	1.69%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 96,513	$ 106,691
Long-term debt, weighted-average interest rate (percent)	2.14%	2.03%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 43,898	$ 48,524
Long-term debt, weighted-average interest rate (percent)	0.61%	0.63%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,073	$ 6,701
Long-term debt, weighted-average interest rate (percent)	3.84%	3.90%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 101,812	$ 115,483
Long-term debt, weighted-average interest rate (percent)	0.85%	0.86%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 87,364	$ 100,089
Long-term debt, weighted-average interest rate (percent)	0.30%	0.35%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 14,051	$ 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	$ 397	$ 416
Long-term debt, weighted-average interest rate (percent)	9.20%	7.75%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,844,699	$ 3,646,164
Long-term debt, weighted-average interest rate (percent)	1.86%	1.88%