Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 5,428		$ 4,972		$ 10,625		$ 9,932
Benefit (provision) for credit losses	3,041		(6,537)	[1]	1,041		(17,091)	[1]
Net interest income (loss) after benefit (provision) for credit losses	8,469		(1,565)		11,666		(7,159)
Guaranty fee income (expense)	58	[2]	50	[2]	120	[2]	100	[2]
Investment gains (losses), net	131		171		247		246
Net other-than-temporary impairments	(599)		(56)		(663)		(100)
Fair value gains (losses), net	(2,449)		(1,634)		(2,166)		(1,345)
Debt extinguishment (losses) gains, net	(93)		(43)		(127)		(30)
Gains from partnership investments	23	[3]	35	[3]	33	[3]	23	[3]
Fee and other income (expense)	337		215		650		402
Administrative expenses	(567)		(569)		(1,131)		(1,174)
Foreclosed property income (expense)	70		478		(269)		(10)
Other (expenses) income	(261)		(67)		(523)		(407)
(Loss) income before federal income taxes	5,119		(2,985)		7,837		(9,454)
Benefit (provision) for federal income taxes	0		93		0		91
Net income (loss)	5,119		(2,892)		7,837		(9,363)
Less: Net (income) loss attributable to the noncontrolling interest	(5)		(1)		(4)		(1)
Net (loss) income attributable to Fannie Mae	5,114		(2,893)		7,833		(9,364)
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(215)		(680)		(594)		(1,578)
Benefit (provision) for credit losses	2,956		(6,414)	[1]	903		(17,032)	[1]
Net interest income (loss) after benefit (provision) for credit losses	2,741		(7,094)		309		(18,610)
Guaranty fee income (expense)	1,970		1,880		3,881		3,751
Investment gains (losses), net	2		(6)		3		(5)
Net other-than-temporary impairments	0		0		0		0
Fair value gains (losses), net	(3)		(3)		(4)		(3)
Debt extinguishment (losses) gains, net	0		0		0		0
Gains from partnership investments	0		0		0		0
Fee and other income (expense)	207		114		407		261
Administrative expenses	(382)		(400)		(762)		(816)
Foreclosed property income (expense)	59		481		(273)		(7)
Other (expenses) income	(240)		(77)		(475)		(395)
(Loss) income before federal income taxes			(5,105)				(15,824)
Benefit (provision) for federal income taxes			109				107
Net income (loss)	4,354		(4,996)		3,086		(15,717)
Less: Net (income) loss attributable to the noncontrolling interest	0		0		0		0
Net (loss) income attributable to Fannie Mae	4,354		(4,996)		3,086		(15,717)
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(6)		(11)		(13)		(20)
Benefit (provision) for credit losses	85		(123)	[1]	138		(59)	[1]
Net interest income (loss) after benefit (provision) for credit losses	79		(134)		125		(79)
Guaranty fee income (expense)	252		216		495		425
Investment gains (losses), net	6		1		12		5
Net other-than-temporary impairments	0		0		0		0
Fair value gains (losses), net	0		0		0		0
Debt extinguishment (losses) gains, net	0		0		0		0
Gains from partnership investments	18		34		29		22
Fee and other income (expense)	49		57		96		115
Administrative expenses	(60)		(64)		(124)		(132)
Foreclosed property income (expense)	11		(3)		4		(3)
Other (expenses) income	3		36		0		42
(Loss) income before federal income taxes			143				395
Benefit (provision) for federal income taxes			(56)				(61)
Net income (loss)	358		87		637		334
Less: Net (income) loss attributable to the noncontrolling interest	0		0		0		0
Net (loss) income attributable to Fannie Mae	358		87		637		334
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	3,443		3,867		6,984		7,577
Benefit (provision) for credit losses	0		0	[1]	0		0	[1]
Net interest income (loss) after benefit (provision) for credit losses	3,443		3,867		6,984		7,577
Guaranty fee income (expense)	(326)		(391)		(658)		(790)
Investment gains (losses), net	1,458		918		2,465		1,788
Net other-than-temporary impairments	(597)		(55)		(661)		(99)
Fair value gains (losses), net	(2,461)		(1,507)		(2,291)		(1,289)
Debt extinguishment (losses) gains, net	(102)		(55)		(172)		(79)
Gains from partnership investments	0		0		0		0
Fee and other income (expense)	186		109		366		184
Administrative expenses	(125)		(105)		(245)		(226)
Foreclosed property income (expense)	0		0		0		0
Other (expenses) income	(3)		(9)		(11)		(18)
(Loss) income before federal income taxes			2,772				7,048
Benefit (provision) for federal income taxes			40				45
Net income (loss)	1,473		2,812		5,777		7,093
Less: Net (income) loss attributable to the noncontrolling interest	0		0		0		0
Net (loss) income attributable to Fannie Mae	1,473		2,812		5,777		7,093
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	1,731	[4]	1,314	[4]	3,300	[4]	2,888	[4]
Benefit (provision) for credit losses	0	[4]	0	[1],[4]	0	[4]	0	[1],[4]
Net interest income (loss) after benefit (provision) for credit losses	1,731	[4]	1,314	[4]	3,300	[4]	2,888	[4]
Guaranty fee income (expense)	(1,206)	[2],[4]	(1,116)	[2],[4]	(2,365)	[2],[4]	(2,226)	[2],[4]
Investment gains (losses), net	87	[4]	(143)	[4]	114	[4]	(169)	[4]
Net other-than-temporary impairments	(2)	[4]	(1)	[4]	(2)	[4]	(1)	[4]
Fair value gains (losses), net	(60)	[4]	(72)	[4]	(8)	[4]	(105)	[4]
Debt extinguishment (losses) gains, net	9	[4]	12	[4]	45	[4]	49	[4]
Gains from partnership investments	0	[4]	0	[4]	0	[4]	0	[4]
Fee and other income (expense)	(100)	[4]	(63)	[4]	(208)	[4]	(155)	[4]
Administrative expenses	0	[4]	0	[4]	0	[4]	0	[4]
Foreclosed property income (expense)	0	[4]	0	[4]	0	[4]	0	[4]
Other (expenses) income	0	[4]	0	[4]	0	[4]	0	[4]
(Loss) income before federal income taxes			(69)	[4]			281	[4]
Benefit (provision) for federal income taxes			0	[4]			0	[4]
Net income (loss)	459	[4]	(69)	[4]	876	[4]	281	[4]
Less: Net (income) loss attributable to the noncontrolling interest	0	[4]	0	[4]	0	[4]	0	[4]
Net (loss) income attributable to Fannie Mae	459	[4]	(69)	[4]	876	[4]	281	[4]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	475	[5],[6]	482	[5],[6]	948	[5],[6]	1,065	[5],[6]
Benefit (provision) for credit losses	0	[5]	0	[1],[5]	0	[5]	0	[1],[5]
Net interest income (loss) after benefit (provision) for credit losses	475	[5]	482	[5]	948	[5]	1,065	[5]
Guaranty fee income (expense)	(632)	[2],[5]	(539)	[2],[5]	(1,233)	[2],[5]	(1,060)	[2],[5]
Investment gains (losses), net	(1,422)	[5],[7]	(599)	[5],[7]	(2,347)	[5],[7]	(1,373)	[5],[7]
Net other-than-temporary impairments	0	[5]	0	[5]	0	[5]	0	[5]
Fair value gains (losses), net	75	[5],[8]	(52)	[5],[8]	137	[5],[8]	52	[5],[8]
Debt extinguishment (losses) gains, net	0	[5]	0	[5]	0	[5]	0	[5]
Gains from partnership investments	5	[5]	1	[5]	4	[5]	1	[5]
Fee and other income (expense)	(5)	[5]	(2)	[5]	(11)	[5]	(3)	[5]
Administrative expenses	0	[5]	0	[5]	0	[5]	0	[5]
Foreclosed property income (expense)	0	[5]	0	[5]	0	[5]	0	[5]
Other (expenses) income	(21)	[5]	(17)	[5]	(37)	[5]	(36)	[5]
(Loss) income before federal income taxes			(726)	[5]			(1,354)	[5]
Benefit (provision) for federal income taxes			0	[5]			0	[5]
Net income (loss)	(1,525)	[5]	(726)	[5]	(2,539)	[5]	(1,354)	[5]
Less: Net (income) loss attributable to the noncontrolling interest	(5)	[5],[9]	(1)	[5],[9]	(4)	[5],[9]	(1)	[5],[9]
Net (loss) income attributable to Fannie Mae	$ (1,530)	[5]	$ (727)	[5]	$ (2,543)	[5]	$ (1,355)	[5]

[1] Prior period amounts have been reclassified to conform to the current period presentation.

[2] Represents the guaranty fees paid from consolidated trusts to the Single-Family and Multifamily segments. The adjustment to guaranty fee income in the Eliminations/Adjustments column represents the elimination of the amortization of deferred cash fees related to consolidated trusts that were re-established for segment reporting. Total guaranty fee income is included in fee and other income in our condensed consolidated statements of operations and comprehensive income (loss).

[3] Gains from partnership investments are included in other expenses in our condensed consolidated statements of operations and comprehensive income (loss).

[4] Represents activity related to the assets and liabilities of consolidated trusts in our condensed consolidated balance sheets.

[5] Represents the elimination of intercompany transactions occurring between the three business segments and our consolidated trusts, as well as other adjustments to reconcile to our consolidated results.

[6] Represents the amortization expense of cost basis adjustments on securities that we own in our portfolio that on a GAAP basis are eliminated.

[7] Primarily represents the removal of realized gains and losses on sales of Fannie Mae MBS classified as available-for-sale securities that are issued by consolidated trusts and retained in the Capital Markets portfolio. The adjustment also includes the removal of securitization gains (losses) recognized in the Capital Markets segment relating to portfolio securitization transactions that do not qualify for sale accounting under GAAP.

[8] Represents the removal of fair value adjustments on consolidated Fannie Mae MBS classified as trading that are retained in the Capital Markets portfolio.

[9] Represents the adjustment from equity method accounting to consolidation accounting for partnership investments that are consolidated in our condensed consolidated balance sheets.