Earnings (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 1,960	$ 3,905	$ 8,488	$ 12,897
Less: Net income attributable to noncontrolling interest	0	0	0	(1)
Net income attributable to Fannie Mae	1,960	3,905	8,488	12,896
Dividends distributed or available for distribution to senior preferred stockholder	(2,202)	(3,999)	(8,357)	(13,403)
Net income (loss) attributable to common stockholders	$ (242)	$ (94)	$ 131	$ (507)
Weighted-average common shares outstanding, Basic	5,762	5,762	5,762	5,762
Convertible preferred stock	0	0	131	0
Weighted-average common shares outstanding, Diluted	5,762	5,762	5,893	5,762
Earnings (loss) per share, Basic (in dollars per share)	$ (0.04)	$ (0.02)	$ 0.02	$ (0.09)
Earnings (loss) per share, Diluted (in dollars per share)	$ (0.04)	$ (0.02)	$ 0.02	$ (0.09)
Weighted average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury	4,600	4,600	4,600	4,600