Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,217,517	$ 4,211,684
Long-term debt, weighted-average interest rate (percent)	2.65%	2.47%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 111,407	$ 123,964
Long-term debt, weighted-average interest rate (percent)	2.68%	2.23%
Unamortized discounts and premiums, fair value adjustments, hedge-related cost basis adjustments, and other cost basis adjustments	$ 5,000	$ 5,100
Fannie Mae | Total senior fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 107,511	$ 118,515
Long-term debt, weighted-average interest rate (percent)	2.41%	1.94%
Fannie Mae | Senior fixed benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 57,341	$ 72,261
Long-term debt, weighted-average interest rate (percent)	2.78%	2.35%
Fannie Mae | Senior fixed medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 43,420	$ 39,476
Long-term debt, weighted-average interest rate (percent)	1.59%	0.78%
Fannie Mae | Senior fixed other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,750	$ 6,778
Long-term debt, weighted-average interest rate (percent)	4.12%	4.00%
Fannie Mae | Total senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,896	$ 5,449
Long-term debt, weighted-average interest rate (percent)	10.73%	8.86%
Fannie Mae | Senior floating Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,656	$ 5,207
Long-term debt, weighted-average interest rate (percent)	10.81%	8.80%
Fannie Mae | Senior floating other debt
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 240	$ 242
Long-term debt, weighted-average interest rate (percent)	9.57%	10.00%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,106,110	$ 4,087,720
Long-term debt, weighted-average interest rate (percent)	2.65%	2.47%