Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 4,851	$ 5,582	$ 5,667	$ 6,304	$ 5,559	$ 5,317	$ 5,428	$ 5,197	$ 22,404		$ 21,501		$ 19,281
Benefit (provision) for credit losses	0	2,609	5,383	957	1,890	(2,079)	3,041	(2,000)	8,949		852		(26,718)
Net interest income (loss) after benefit (provision) for credit losses	4,851	8,191	11,050	7,261	7,449	3,238	8,469	3,197	31,353		22,353		(7,437)
Guaranty fee income (expense)									205	[1],[2]	212	[1],[2]	227	[1]
Investment gains, net	135	648	290	118	106	134	131	116	1,191		487		506
Net other-than-temporary impairments	(22)	(27)	(6)	(9)	(12)	(38)	(599)	(64)	(64)	[3]	(713)		(308)
Fair value gains (losses), net	961	335	829	834	209	(1,020)	(2,449)	283	2,959		(2,977)		(6,621)
Debt extinguishment gains (losses), net	35	92	27	(23)	(63)	(54)	(93)	(34)	131		(244)		(232)
Gains (losses) from partnership investments									517	[4]	119	[4]	81	[4]
Fee and other income (expense)									3,725		1,275		936
Administrative expenses	(632)	(646)	(626)	(641)	(648)	(588)	(567)	(564)	(2,545)		(2,367)		(2,370)
Foreclosed property income (expense)	1,082	1,165	332	260	475	48	70	(339)	2,839		254		(780)
TCCA fees									(1,001)		(238)		0
Other (expenses) income									(743)		(941)	[5]	(947)
Income (loss) before federal income taxes	8,274	10,099	12,080	8,114					38,567		17,220		(16,945)
(Provision) benefit for federal income taxes	(1,816)	(1,355)	(1,985)	50,571					45,415	[6]	0		90
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	6,458	8,744	10,095	58,685	7,570	1,813	5,119	2,718	83,982		17,220		(16,855)
Less: Net (income) loss attributable to noncontrolling interest	(1)	(7)	(11)	0	0	8	(5)	1	(19)		4		0
Net income (loss) attributable to Fannie Mae	6,457	8,737	10,084	58,685	7,570	1,821	5,114	2,719	83,963		17,224		(16,855)
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income									205		(790)		(2,411)
Benefit (provision) for credit losses									8,469		672		(26,453)
Net interest income (loss) after benefit (provision) for credit losses									8,674		(118)		(28,864)
Guaranty fee income (expense)									10,468	[2]	8,151	[2]	7,507
Investment gains, net									3		8		(2)
Net other-than-temporary impairments									0		0		0
Fair value gains (losses), net									(10)		(8)		(7)
Debt extinguishment gains (losses), net									0		0		0
Gains (losses) from partnership investments									0	[4]	0	[4]	0	[4]
Fee and other income (expense)									630		759		579
Administrative expenses									(1,706)		(1,590)		(1,638)
Foreclosed property income (expense)									2,736		247		(765)
Other (expenses) income									(628)		(841)	[5]	(857)
Income (loss) before federal income taxes									19,166		6,370		(24,047)
(Provision) benefit for federal income taxes									29,110	[6]	(80)		106
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									48,276		6,290
Less: Net (income) loss attributable to noncontrolling interest									0		0
Net income (loss) attributable to Fannie Mae									48,276		6,290		(23,941)
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income									(74)		(13)		(38)
Benefit (provision) for credit losses									480		180		(265)
Net interest income (loss) after benefit (provision) for credit losses									406		167		(303)
Guaranty fee income (expense)									1,217	[2]	1,040	[2]	884
Investment gains, net									21		37		18
Net other-than-temporary impairments									0		0		0
Fair value gains (losses), net									0		0		0
Debt extinguishment gains (losses), net									0		0		0
Gains (losses) from partnership investments									498	[4]	123	[4]	81	[4]
Fee and other income (expense)									182		207		218
Administrative expenses									(280)		(269)		(264)
Foreclosed property income (expense)									103		7		(15)
Other (expenses) income									(2)		(5)	[5]	25
Income (loss) before federal income taxes									2,145		1,307		644
(Provision) benefit for federal income taxes									7,924	[6]	204		(61)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									10,069		1,511
Less: Net (income) loss attributable to noncontrolling interest									0		0
Net income (loss) attributable to Fannie Mae									10,069		1,511		583
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income									9,764		13,241		13,920
Benefit (provision) for credit losses									0		0		0
Net interest income (loss) after benefit (provision) for credit losses									9,764		13,241		13,920
Guaranty fee income (expense)									(1,115)	[2]	(1,291)	[2]	(1,497)
Investment gains, net									4,911		6,217		3,711
Net other-than-temporary impairments									(64)		(711)		(306)
Fair value gains (losses), net									3,148		(3,041)		(6,596)
Debt extinguishment gains (losses), net									27		(277)		(254)
Gains (losses) from partnership investments									0	[4]	0	[4]	0	[4]
Fee and other income (expense)									3,010		717		478
Administrative expenses									(559)		(508)		(468)
Foreclosed property income (expense)									0		0		0
Other (expenses) income									20		(22)	[5]	(34)
Income (loss) before federal income taxes									19,142		14,325		8,954
(Provision) benefit for federal income taxes									8,381	[6]	(124)		45
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									27,523		14,201
Less: Net (income) loss attributable to noncontrolling interest									0		0
Net income (loss) attributable to Fannie Mae									27,523		14,201		8,999
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(306)	(276)	(233)	(186)	(134)	(78)	(26)	0	(1,001)	[2]	(238)	[2],[5]
US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
TCCA fees									(1,001)	[2]	(238)	[2],[5]
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income									10,939	[7]	7,156	[7]	5,765	[7]
Benefit (provision) for credit losses									0	[7]	0	[7]	0	[7]
Net interest income (loss) after benefit (provision) for credit losses									10,939	[7]	7,156	[7]	5,765	[7]
Guaranty fee income (expense)									(5,233)	[1],[2],[7]	(4,737)	[1],[2],[7]	(4,486)	[1],[7]
Investment gains, net									(122)	[7]	(1)	[7]	(315)	[7]
Net other-than-temporary impairments									0	[7]	(2)	[7]	(2)	[7]
Fair value gains (losses), net									(722)	[7]	(313)	[7]	(226)	[7]
Debt extinguishment gains (losses), net									104	[7]	33	[7]	22	[7]
Gains (losses) from partnership investments									0	[4],[7]	0	[4],[7]	0	[4],[7]
Fee and other income (expense)									(321)	[7]	(395)	[7]	(329)	[7]
Administrative expenses									0	[7]	0	[7]	0	[7]
Foreclosed property income (expense)									0	[7]	0	[7]	0	[7]
Other (expenses) income									0	[7]	0	[5],[7]	0	[7]
Income (loss) before federal income taxes									4,645	[7]	1,741	[7]	429	[7]
(Provision) benefit for federal income taxes									0	[6],[7]	0	[7]	0	[7]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									4,645	[7]	1,741	[7]
Less: Net (income) loss attributable to noncontrolling interest									0	[7]	0	[7]
Net income (loss) attributable to Fannie Mae									4,645	[7]	1,741	[7]	429	[7]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income									1,570	[8],[9]	1,907	[8],[9]	2,045	[8],[9]
Benefit (provision) for credit losses									0	[8]	0	[8]	0	[8]
Net interest income (loss) after benefit (provision) for credit losses									1,570	[8]	1,907	[8]	2,045	[8]
Guaranty fee income (expense)									(5,132)	[1],[2],[8]	(2,951)	[1],[2],[8]	(2,181)	[1],[8]
Investment gains, net									(3,622)	[10],[8]	(5,774)	[10],[8]	(2,906)	[10],[8]
Net other-than-temporary impairments									0	[8]	0	[8]	0	[8]
Fair value gains (losses), net									543	[11],[8]	385	[11],[8]	208	[11],[8]
Debt extinguishment gains (losses), net									0	[8]	0	[8]	0	[8]
Gains (losses) from partnership investments									19	[4],[8]	(4)	[4],[8]	0	[4],[8]
Fee and other income (expense)									224	[8]	(13)	[8]	(10)	[8]
Administrative expenses									0	[8]	0	[8]	0	[8]
Foreclosed property income (expense)									0	[8]	0	[8]	0	[8]
Other (expenses) income									(133)	[8]	(73)	[5],[8]	(81)	[8]
Income (loss) before federal income taxes									(6,531)	[8]	(6,523)	[8]	(2,925)	[8]
(Provision) benefit for federal income taxes									0	[6],[8]	0	[8]	0	[8]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									(6,531)	[8]	(6,523)	[8]
Less: Net (income) loss attributable to noncontrolling interest									(19)	[12],[8]	4	[12],[8]
Net income (loss) attributable to Fannie Mae									$ (6,550)	[8]	$ (6,519)	[8]	$ (2,925)	[8]

[1] 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 related to unconsolidated Fannie Mae MBS trusts and other credit enhancement arrangements is included in fee and other income in our consolidated statements of operations and comprehensive income (loss).

[2] Pursuant to the TCCA, effective April 1, 2012, we increased the guaranty fee on all single-family residential mortgages delivered to us on or after that date by 10 basis points, and the incremental revenue must be remitted to Treasury. The resulting revenue is included in guaranty fee income and the expense is recognized as “TCCA fees.” This increase in guaranty fee is also included in the single-family average charged guaranty fee.

[3] Includes $34 million of other-than-temporary impairments recognized in earnings for the year ended December 31, 2013, related to our intent to sell the related securities before recovery of their amortized cost basis.

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

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

[6] Primarily represents the release of the valuation allowance for our deferred tax assets that generally are directly attributable to each segment based on the nature of the item.

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

[8] 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.

[9] Represents the amortization expense of cost basis adjustments on securities in the Capital Markets group’s retained mortgage portfolio that on a GAAP basis are eliminated.

[10] 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 in the Capital Markets group’s retained mortgage 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.

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

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