Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,216,799	$ 4,211,684
Long-term debt, weighted-average interest rate (percent)	2.55%	2.47%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 125,197	$ 123,964
Long-term debt, weighted-average interest rate (percent)	2.32%	2.23%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 4,600	$ 5,100
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 119,843	$ 118,515
Long-term debt, weighted-average interest rate (percent)	2.00%	1.94%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 70,682	$ 72,261
Long-term debt, weighted-average interest rate (percent)	2.36%	2.35%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 42,344	$ 39,476
Long-term debt, weighted-average interest rate (percent)	1.03%	0.78%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,817	$ 6,778
Long-term debt, weighted-average interest rate (percent)	4.03%	4.00%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,354	$ 5,449
Long-term debt, weighted-average interest rate (percent)	9.75%	8.86%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,104	$ 5,207
Long-term debt, weighted-average interest rate (percent)	9.75%	8.80%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 250	$ 242
Long-term debt, weighted-average interest rate (percent)	9.68%	10.00%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,091,602	$ 4,087,720
Long-term debt, weighted-average interest rate (percent)	2.56%	2.47%