Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 2,467	$ 1,960	$ 4,640	$ 1,888	$ 1,312	$ 3,905	$ 3,667	$ 5,325	$ 10,955	$ 14,209	$ 83,982
Less: Net income attributable to noncontrolling interest	(1)	0	0	0	0	0	(1)	0	(1)	(1)	(19)
Net income attributable to Fannie Mae	2,466	1,960	4,640	1,888	1,312	3,905	3,666	5,325	10,954	14,208	83,963
Dividends distributed or available for distribution to senior preferred stockholder	(2,859)	(2,202)	(4,359)	(1,796)	(1,920)	(3,999)	(3,712)	(5,692)	(11,216)	(15,323)	(85,419)
Net loss attributable to common stockholders (Note 11)	$ (393)	$ (242)	$ 281	$ 92	$ (608)	$ (94)	$ (46)	$ (367)	$ (262)	$ (1,115)	$ (1,456)
Weighted-average common shares outstanding: Basic and Diluted					5,762	5,762	5,762	5,762	5,762	5,762	5,762
Loss per share: Basic and Diluted (in dollars per share)	$ (0.07)	$ (0.04)	$ 0.05	$ 0.02	$ (0.11)	$ (0.02)	$ (0.01)	$ (0.06)	$ (0.05)	$ (0.19)	$ (0.25)
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