Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Earnings Per Share [Abstract]
Net income (loss)	$ 7,570	$ 1,813	$ 5,119	$ 2,718	$ (2,407)	[1]	$ (5,085)	$ (2,892)	$ (6,471)	$ 17,220		$ (16,855)		$ (14,018)
Less: Net loss attributable to noncontrolling interest	0	8	(5)	1	1		0	(1)	0	4		0		4
Net income (loss) attributable to Fannie Mae	7,570	1,821	5,114	2,719	(2,406)		(5,085)	(2,893)	(6,471)	17,224		(16,855)		(14,014)
Preferred stock dividends	(2,928)	(2,929)	(2,929)	(2,817)	(2,622)		(2,494)	(2,282)	(2,216)	(11,603)		(9,614)		(7,704)
Undistributed earnings available for distribution to senior preferred stockholders	(4,224)	0	0	0						(4,224)		0		0
Net income (loss) attributable to common stockholders	$ 418	$ (1,108)	$ 2,185	$ (98)	$ (5,028)		$ (7,579)	$ (5,175)	$ (8,687)	$ 1,397		$ (26,469)		$ (21,718)
Weighted-average common shares outstanding—Basic	5,762,000,000	5,762,000,000	5,762,000,000	5,761,000,000						5,762,000,000	[2]	5,737,000,000	[2]	5,694,000,000	[2]
Convertible preferred stock										131,000,000		0		0
Weighted-average common shares outstanding—Diluted	5,893,000,000	5,762,000,000	5,893,000,000	5,761,000,000						5,893,000,000	[2]	5,737,000,000	[2]	5,694,000,000	[2]
Earnings (loss) per share:
Basic	$ 0.07	$ (0.19)	$ 0.38	$ (0.02)						$ 0.24		$ (4.61)		$ (3.81)
Diluted	$ 0.07	$ (0.19)	$ 0.37	$ (0.02)						$ 0.24		$ (4.61)		$ (3.81)
Antidilutive weighted-average options and performance awards excluded from computation of diluted EPS										3,000,000		5,000,000		8,000,000
Weighted-average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury										4,700,000,000		4,600,000,000		4,600,000,000

[1] Includes an out-of-period adjustment of $933 million comprised of $1.2 billion to reduce “Interest Income: Available-for-sale securities” offset by a $264 million reduction to “Other-than-temporary impairments” in our consolidated statement of operations and comprehensive loss for the three months ended December 31, 2011.

[2] Includes 4.7 billion for the year ended December 31, 2012, and 4.6 billion for both the years ended December 31, 2011 and 2010 of weighted-average shares of common stock, that would be issued upon the full exercise of the warrant issued to Treasury from the date the warrant was issued through December 31, 2012, 2011 and 2010, respectively.