Selected Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per 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
Interest income:
Trading securities	$ 233		$ 234		$ 73		$ 449		$ 265		$ 274	$ 264	$ 284	$ 989		$ 1,087		$ 1,251
Available-for-sale securities	748		789		1,035		727		(248)	[1]	1,160	1,152	1,213	3,299		3,277		5,290
Mortgage loans	29,520		30,593		32,023		32,570		33,205		34,334	35,333	35,590	124,706		138,462		147,583
Other	65		53		40		38		38		26	25	28	196		117		146
Total interest income	30,566		31,669		33,171		33,784		33,260		35,794	36,774	37,115	129,190		142,943		154,270
Interest expense:
Short-term debt	40		38		32		42		56		66	81	107	152		310		631
Long-term debt	24,967		26,314		27,711		28,545		29,041		30,542	31,721	32,048	107,537		123,352		137,230
Total interest expense	25,007		26,352		27,743		28,587		29,097		30,608	31,802	32,155	107,689		123,662		137,861
Net interest income	5,559		5,317		5,428		5,197		4,163		5,186	4,972	4,960	21,501		19,281		16,409
Benefit (provision) for credit losses	1,890	[2]	(2,079)	[2]	3,041	[2]	(2,000)	[2]	(5,476)		(4,151)	(6,537)	(10,554)	852		(26,718)		(24,896)
Net interest income (loss) after benefit (provision) for credit losses	7,449		3,238		8,469		3,197		(1,313)		1,035	(1,565)	(5,594)	22,353		(7,437)		(8,487)
Investment gains, net	106		134		131		116		187		73	171	75	487		506		346
Other-than-temporary impairments	(18)		(17)		(196)		(80)		(297)	[1]	(232)	(28)	(57)	(311)		(614)		(694)
Noncredit portion of other-than-temporary impairments recognized in other comprehensive income (loss)	6		(21)		(403)		16		351		(30)	(28)	13	(402)		306		(28)
Net other-than-temporary impairments	(12)		(38)		(599)		(64)		54		(262)	(56)	(44)	(713)		(308)		(722)	[3]
Fair value (losses) gains, net	209		(1,020)		(2,449)		283		(751)		(4,525)	(1,634)	289	(2,977)		(6,621)		(511)
Debt extinguishment (losses) gains, net	(63)		(54)		(93)		(34)		(83)		(119)	(43)	13	(244)		(232)		(568)
Fee and other income	339		378		395		375		370		291	265	237	1,487		1,163		1,084
Non-interest income (loss)	579		(600)		(2,615)		676		(223)		(4,542)	(1,297)	570	(1,960)		(5,492)		(371)
Administrative expenses:
Salaries and employee benefits	303		294		292		306		283		323	310	320	1,195		1,236		1,277
Professional services	224		195		179		168		205		173	169	189	766		736		942
Occupancy expenses	49		48		48		43		48		46	43	42	188		179		170
Other administrative expenses	72		51		48		47		69		49	47	54	218		219		208
Total administrative expenses	648		588		567		564		605		591	569	605	2,367		2,370		2,597
Foreclosed property (income) expense	(475)		(48)		(70)		339		37		733	(478)	488	(254)		780		1,718
Other expenses	285		285		238		252		228		254	32	352	1,060		866		927
Total expenses	458		825		735		1,155		870		1,578	123	1,445	3,173		4,016		5,242
Income (loss) before federal income taxes									(2,406)		(5,085)	(2,985)	(6,469)	17,220		(16,945)		(14,100)
(Provision) benefit for federal income taxes									(1)		0	93	(2)	0		90		82
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 (income) 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 (loss) income attributable to common stockholders (Note 11)	$ 418		$ (1,108)		$ 2,185		$ (98)		$ (5,028)		$ (7,579)	$ (5,175)	$ (8,687)	$ 1,397		$ (26,469)		$ (21,718)
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)
Loss per share—Basic and Diluted									$ (0.87)		$ (1.32)	$ (0.90)	$ (1.52)
Weighted-average common shares outstanding:
Basic	5,762		5,762		5,762		5,761							5,762	[4]	5,737	[4]	5,694	[4]
Diluted	5,893		5,762		5,893		5,761							5,893	[4]	5,737	[4]	5,694	[4]
Weighted-average common shares outstanding—Basic and Diluted									5,760		5,760	5,730	5,698

[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 out-of-period adjustments of $548 million, $503 million, $850 million, and $172 million which increased our provision for credit losses for the three months ended March 31, 2012, June 30, 2012, September 30, 2012, and December 31, 2012, respectively.

[3] Certain prior period amounts have been reclassified to conform to the current period presentation.

[4] 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.