Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,677,725	$ 3,440,724
Long-term debt, weighted-average interest rate	2.30%	2.78%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,444,338	$ 3,285,139
Long-term debt, weighted-average interest rate	2.34%	2.78%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 233,387	$ 155,585
Long-term debt, weighted-average interest rate	1.68%	2.86%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 430	$ 2
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 114,586	$ 123,958
Long-term debt, weighted-average interest rate	2.44%	2.47%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 91,587	$ 86,114
Long-term debt, weighted-average interest rate	2.53%	2.66%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 19,263	$ 32,590
Long-term debt, weighted-average interest rate	1.26%	1.57%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,736	$ 5,254
Long-term debt, weighted-average interest rate	6.10%	5.01%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 118,778	$ 31,596
Long-term debt, weighted-average interest rate	0.95%	4.40%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 100,556	$ 9,774
Long-term debt, weighted-average interest rate	0.34%	1.66%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 17,789	$ 21,424
Long-term debt, weighted-average interest rate	4.31%	5.61%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 433	$ 398
Long-term debt, weighted-average interest rate	5.09%	6.27%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 23	$ 31
Long-term debt, weighted-average interest rate	2.80%	2.31%