Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,440,724	$ 3,367,024
Long-term debt, weighted-average interest rate	2.78%	3.02%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,285,139	$ 3,159,846
Long-term debt, weighted-average interest rate	2.78%	3.03%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 155,585	$ 207,178
Long-term debt, weighted-average interest rate	2.86%	2.83%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 2	$ 413
Fannie Mae [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Medium-term Notes Original Contractual Maturity 1	1 year
Long-term debt, original contractual maturity greater than 10 years	10 years
Fannie Mae [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Medium-term Notes Original Contractual Maturity 1	10 years
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 123,958	$ 171,344
Long-term debt, weighted-average interest rate	2.47%	2.13%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 86,114	$ 103,206
Long-term debt, weighted-average interest rate	2.66%	2.36%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 32,590	$ 61,455
Long-term debt, weighted-average interest rate	1.57%	1.48%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,254	$ 6,683
Long-term debt, weighted-average interest rate	5.01%	4.62%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 31,596	$ 30,166
Long-term debt, weighted-average interest rate	4.40%	5.52%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 9,774	$ 4,174
Long-term debt, weighted-average interest rate	1.66%	2.36%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 21,424	$ 25,641
Long-term debt, weighted-average interest rate	5.61%	5.97%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 398	$ 351
Long-term debt, weighted-average interest rate	6.27%	10.19%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 0	$ 5,617
Long-term debt, weighted-average interest rate	0.00%	9.64%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 31	$ 51
Long-term debt, weighted-average interest rate	2.31%	1.96%