(Loss) Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 1,312	$ 3,905	$ 3,667	$ 5,325	$ 6,458	$ 8,744	$ 10,095	$ 58,685	$ 14,209	$ 83,982	$ 17,220
Less: Net (income) loss attributable to noncontrolling interest	0	0	(1)	0	(1)	(7)	(11)	0	(1)	(19)	4
Net income attributable to Fannie Mae	1,312	3,905	3,666	5,325	6,457	8,737	10,084	58,685	14,208	83,963	17,224
Dividends distributed or available for distribution to senior preferred stockholder	(1,920)	(3,999)	(3,712)	(5,692)	(7,191)	(8,617)	(10,243)	(59,368)	(15,323)	(85,419)	(15,827)
Net (loss) income attributable to common stockholders (Note 11)	$ (608)	$ (94)	$ (46)	$ (367)	$ (734)	$ 120	$ (159)	$ (683)	$ (1,115)	$ (1,456)	$ 1,397
Weighted-average common shares outstanding—Basic					5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000
Convertible preferred stock									0	0	131,000,000
Weighted-average common shares outstanding—Diluted					5,762,000,000	5,893,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,893,000,000
Earnings (loss) per share:
Basic (per share)					$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ (0.19)	$ (0.25)	$ 0.24
Diluted (per share)					$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ (0.19)	$ (0.25)	$ 0.24
Senior Preferred Stock, Dividend Rate, Percentage											10.00%
Weighted-average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury									4,600,000,000	4,600,000,000	4,700,000,000