Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,923,563	$ 3,440,724
Long-term debt, weighted-average interest rate	1.85%	2.78%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 277,399	$ 155,585
Long-term debt, weighted-average interest rate	1.34%	2.86%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 392	$ 2
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 161,916	$ 123,958
Long-term debt, weighted-average interest rate	1.69%	2.47%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,701	$ 5,254
Long-term debt, weighted-average interest rate	3.90%	5.01%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 48,524	$ 32,590
Long-term debt, weighted-average interest rate	0.63%	1.57%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 106,691	$ 86,114
Long-term debt, weighted-average interest rate	2.03%	2.66%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 115,483	$ 31,596
Long-term debt, weighted-average interest rate	0.86%	4.40%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 14,978	$ 21,424
Long-term debt, weighted-average interest rate	4.16%	5.61%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 416	$ 398
Long-term debt, weighted-average interest rate	7.75%	6.27%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 100,089	$ 9,774
Long-term debt, weighted-average interest rate	0.35%	1.66%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 0	$ 31
Long-term debt, weighted-average interest rate	0.00%	2.31%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,646,164	$ 3,285,139
Long-term debt, weighted-average interest rate	1.88%	2.78%