Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,205,404	$ 4,211,684
Long-term debt, weighted-average interest rate	2.57%	2.47%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 106,751	$ 123,964
Long-term debt, weighted-average interest rate	2.63%	2.23%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 4,000	$ 5,100
Fannie Mae | Senior Fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 103,731	$ 118,515
Long-term debt, weighted-average interest rate	2.40%	1.94%
Fannie Mae | Benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 54,727	$ 72,261
Long-term debt, weighted-average interest rate	2.79%	2.35%
Fannie Mae | Medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 42,217	$ 39,476
Long-term debt, weighted-average interest rate	1.58%	0.78%
Fannie Mae | Other
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,787	$ 6,778
Long-term debt, weighted-average interest rate	3.98%	4.00%
Fannie Mae | Senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,020	$ 5,449
Long-term debt, weighted-average interest rate	10.92%	8.86%
Fannie Mae | Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,752	$ 5,207
Long-term debt, weighted-average interest rate	11.12%	8.80%
Fannie Mae | Other
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 268	$ 242
Long-term debt, weighted-average interest rate	8.79%	10.00%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,098,653	$ 4,087,720
Long-term debt, weighted-average interest rate	2.56%	2.47%