Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,393,678	$ 3,367,024
Long-term debt, weighted-average interest rate	2.91%	3.02%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,199,765	$ 3,159,846
Long-term debt, weighted-average interest rate	2.90%	3.03%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 193,913	$ 207,178
Long-term debt, weighted-average interest rate	3.03%	2.83%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 151	$ 413
Fannie Mae [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	1 year
Long-term debt original contractual maturity greater than 10 years	10 years	10 years
Fannie Mae [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years	10 years
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 155,850	$ 171,344
Long-term debt, weighted-average interest rate	2.31%	2.13%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 97,884	$ 103,206
Long-term debt, weighted-average interest rate	2.56%	2.36%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 51,345	$ 61,455
Long-term debt, weighted-average interest rate	1.53%	1.48%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,621	$ 6,683
Long-term debt, weighted-average interest rate	4.80%	4.62%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 32,381	$ 30,166
Long-term debt, weighted-average interest rate	5.25%	5.52%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,773	$ 4,174
Long-term debt, weighted-average interest rate	2.50%	2.36%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 24,210	$ 25,641
Long-term debt, weighted-average interest rate	6.12%	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	6.37%	10.19%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,641	$ 5,617
Long-term debt, weighted-average interest rate	9.99%	9.64%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 41	$ 51
Long-term debt, weighted-average interest rate	2.06%	1.96%