Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,155,396	$ 3,923,563
Long-term debt, weighted-average interest rate	1.88%	1.85%
Fannie Mae
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 198,097	$ 277,399
Long-term debt, weighted-average interest rate	1.55%	1.34%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 1,600	$ 392
Fannie Mae | Senior Fixed
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 134,975	$ 161,916
Long-term debt, weighted-average interest rate	1.78%	1.69%
Fannie Mae | Benchmark notes and bonds
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 89,618	$ 106,691
Long-term debt, weighted-average interest rate	2.13%	2.03%
Fannie Mae | Medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 38,312	$ 48,524
Long-term debt, weighted-average interest rate	0.60%	0.63%
Fannie Mae | Other
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,045	$ 6,701
Long-term debt, weighted-average interest rate	3.73%	3.90%
Fannie Mae | Senior floating
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 63,122	$ 115,483
Long-term debt, weighted-average interest rate	1.05%	0.86%
Fannie Mae | Medium-term notes
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 51,583	$ 100,089
Long-term debt, weighted-average interest rate	0.32%	0.35%
Fannie Mae | Connecticut Avenue Securities
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 11,166	$ 14,978
Long-term debt, weighted-average interest rate	4.30%	4.16%
Fannie Mae | Other
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 373	$ 416
Long-term debt, weighted-average interest rate	7.17%	7.75%
Consolidated Trusts
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,957,299	$ 3,646,164
Long-term debt, weighted-average interest rate	1.89%	1.88%