Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,296,298	$ 3,226,737
Long-term debt, weighted-average interest rate	2.77%	2.54%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,052,923	$ 2,934,635
Long-term debt, weighted-average interest rate	2.80%	2.57%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 243,375	$ 292,102
Long-term debt, weighted-average interest rate	2.40%	2.31%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 752	$ 1,800
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	$ 206,863	$ 249,013
Long-term debt, weighted-average interest rate	1.95%	2.14%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 123,541	$ 153,983
Long-term debt, weighted-average interest rate	2.11%	2.16%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 75,901	$ 82,230
Long-term debt, weighted-average interest rate	1.41%	1.40%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,421	$ 12,800
Long-term debt, weighted-average interest rate	4.84%	6.74%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 31,328	$ 38,333
Long-term debt, weighted-average interest rate	4.14%	2.48%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 8,425	$ 21,476
Long-term debt, weighted-average interest rate	1.36%	0.71%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 22,527	$ 16,511
Long-term debt, weighted-average interest rate	5.18%	4.77%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 376	$ 346
Long-term debt, weighted-average interest rate	6.36%	6.75%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,106	$ 4,645
Long-term debt, weighted-average interest rate	9.93%	9.93%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 78	$ 111
Long-term debt, weighted-average interest rate	1.70%	1.44%