Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,204,752	$ 4,155,396
Long-term debt, weighted-average interest rate (percent)	2.02%	1.88%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 176,124	$ 198,097
Long-term debt, weighted-average interest rate (percent)	1.68%	1.55%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 3,200	$ 1,600
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 128,001	$ 134,975
Long-term debt, weighted-average interest rate (percent)	1.81%	1.78%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 82,954	$ 89,618
Long-term debt, weighted-average interest rate (percent)	2.18%	2.13%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 38,114	$ 38,312
Long-term debt, weighted-average interest rate (percent)	0.60%	0.60%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,933	$ 7,045
Long-term debt, weighted-average interest rate (percent)	3.79%	3.73%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 48,123	$ 63,122
Long-term debt, weighted-average interest rate (percent)	1.34%	1.05%
Fannie Mae | Senior floating medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 37,410	$ 51,583
Long-term debt, weighted-average interest rate (percent)	0.42%	0.32%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 10,406	$ 11,166
Long-term debt, weighted-average interest rate (percent)	4.51%	4.30%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 307	$ 373
Long-term debt, weighted-average interest rate (percent)	7.77%	7.17%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,028,628	$ 3,957,299
Long-term debt, weighted-average interest rate (percent)	2.03%	1.89%