Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,504,219	$ 3,440,724
Long-term debt, weighted-average interest rate	2.76%	2.78%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,334,098	$ 3,285,139
Long-term debt, weighted-average interest rate	2.78%	2.78%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 170,121	$ 155,585
Long-term debt, weighted-average interest rate	2.43%	2.86%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 586	$ 2
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 110,114	$ 123,958
Long-term debt, weighted-average interest rate	2.57%	2.47%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 83,115	$ 86,114
Long-term debt, weighted-average interest rate	2.70%	2.66%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 22,919	$ 32,590
Long-term debt, weighted-average interest rate	1.59%	1.57%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,080	$ 5,254
Long-term debt, weighted-average interest rate	5.47%	5.01%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 59,980	$ 31,596
Long-term debt, weighted-average interest rate	2.17%	4.40%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 40,078	$ 9,774
Long-term debt, weighted-average interest rate	0.54%	1.66%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 19,470	$ 21,424
Long-term debt, weighted-average interest rate	5.42%	5.61%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 432	$ 398
Long-term debt, weighted-average interest rate	7.37%	6.27%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 27	$ 31
Long-term debt, weighted-average interest rate	2.60%	2.31%