Segment Reporting Segment Reporting(Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					9 Months Ended
	Sep. 30, 2013		Mar. 31, 2013	Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 5,582			$ 5,317		$ 17,553		$ 15,942
Benefit (provision) for credit losses	2,609			(2,079)		8,949		(1,038)
Net interest income (loss) after benefit (provision) for credit losses	8,191			3,238		26,502		14,904
Guaranty Fee Income	51	[1]		56	[1]	156	[1]	176	[1]
Investment gains (losses), net	648			134		1,056		381
Net other-than-temporary impairments	(27)	[2]		(38)		(42)	[2]	(701)
Fair value gains (losses), net	335			(1,020)		1,998		(3,186)
Debt extinguishment (losses) gains, net	92			(54)		96		(181)
Gains (losses) from partnership investments	128	[3]		35	[3]	302	[3]	68	[3]
Fee and other income (expense)	690			322		1,638		972
Administrative expenses	(646)			(588)		(1,913)		(1,719)
Foreclosed property income (expense)	1,165			48		1,757		(221)
Other (expenses) income	(528)			(320)		(1,257)		(843)
Income (loss) before federal income taxes	10,099			1,813		30,293		9,650
Benefit (Provision) for federal income taxes	(1,355)		50,600	0		47,231	[4]	0
Net income (loss)	8,744			1,813		77,524		9,650
Less: Net (income) loss attributable to noncontrolling interest	(7)			8		(18)		4
Net (loss) income attributable to Fannie Mae	8,737			1,821		77,506		9,654
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(152)			(192)		318		(786)
Benefit (provision) for credit losses	2,471			(2,176)		8,605		(1,273)
Net interest income (loss) after benefit (provision) for credit losses	2,319			(2,368)		8,923		(2,059)
Guaranty Fee Income	2,719			2,014		7,638		5,895
Investment gains (losses), net	1			5		4		8
Net other-than-temporary impairments	0			0		0		0
Fair value gains (losses), net	(2)			(1)		(5)		(5)
Debt extinguishment (losses) gains, net	0			0		0		0
Gains (losses) from partnership investments	0			0		0		0
Fee and other income (expense)	151			181		502		588
Administrative expenses	(436)			(397)		(1,281)		(1,159)
Foreclosed property income (expense)	1,171			46		1,752		(227)
Other (expenses) income	(427)			(290)		(1,168)		(765)
Income (loss) before federal income taxes	5,496			(810)		16,365		2,276
Benefit (Provision) for federal income taxes	(751)			(12)		29,777	[4]	(12)
Net income (loss)	4,745			(822)		46,142		2,264
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	4,745			(822)		46,142		2,264
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(31)			(1)		(59)		(14)
Benefit (provision) for credit losses	138			97		344		235
Net interest income (loss) after benefit (provision) for credit losses	107			96		285		221
Guaranty Fee Income	311			265		902		760
Investment gains (losses), net	3			11		14		23
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 (losses) from partnership investments	121			43		284		72
Fee and other income (expense)	26			55		115		151
Administrative expenses	(73)			(70)		(210)		(194)
Foreclosed property income (expense)	(6)			2		5		6
Other (expenses) income	(3)			(7)		(2)		(7)
Income (loss) before federal income taxes	486			395		1,393		1,032
Benefit (Provision) for federal income taxes	(8)			32		7,970	[4]	32
Net income (loss)	478			427		9,363		1,064
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	478			427		9,363		1,064
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	2,311			3,247		7,733		10,231
Benefit (provision) for credit losses	0			0		0		0
Net interest income (loss) after benefit (provision) for credit losses	2,311			3,247		7,733		10,231
Guaranty Fee Income	(273)			(319)		(855)		(977)
Investment gains (losses), net	1,590			2,201		3,837		4,666
Net other-than-temporary impairments	(27)			(38)		(42)		(699)
Fair value gains (losses), net	371			(961)		2,087		(3,252)
Debt extinguishment (losses) gains, net	54			(46)		17		(218)
Gains (losses) from partnership investments	0			0		0		0
Fee and other income (expense)	525			185		1,129		551
Administrative expenses	(137)			(121)		(422)		(366)
Foreclosed property income (expense)	0			0		0		0
Other (expenses) income	(19)			(6)		31		(17)
Income (loss) before federal income taxes	4,395			4,142		13,515		9,919
Benefit (Provision) for federal income taxes	(596)			(20)		9,484	[4]	(20)
Net income (loss)	3,799			4,122		22,999		9,899
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	3,799			4,122		22,999		9,899
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	3,085	[5]		1,778	[5]	8,303	[5]	5,078	[5]
Benefit (provision) for credit losses	0	[5]		0	[5]	0	[5]	0	[5]
Net interest income (loss) after benefit (provision) for credit losses	3,085	[5]		1,778	[5]	8,303	[5]	5,078	[5]
Guaranty Fee Income	(1,345)	[1],[5]		(1,219)	[1],[5]	(3,832)	[1],[5]	(3,584)	[1],[5]
Investment gains (losses), net	77	[5]		(64)	[5]	(79)	[5]	50	[5]
Net other-than-temporary impairments	0	[5]		0	[5]	0	[5]	(2)	[5]
Fair value gains (losses), net	(120)	[5]		(58)	[5]	(538)	[5]	(66)	[5]
Debt extinguishment (losses) gains, net	38	[5]		(8)	[5]	79	[5]	37	[5]
Gains (losses) from partnership investments	0	[5]		0	[5]	0	[5]	0	[5]
Fee and other income (expense)	(84)	[5]		(94)	[5]	(254)	[5]	(302)	[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	0	[5]		0	[5]	0	[5]	0	[5]
Income (loss) before federal income taxes	1,651	[5]		335	[5]	3,679	[5]	1,211	[5]
Benefit (Provision) for federal income taxes	0	[5]		0	[5]	0	[4],[5]	0	[5]
Net income (loss)	1,651	[5]		335	[5]	3,679	[5]	1,211	[5]
Less: Net (income) loss attributable to noncontrolling interest	0	[5]		0	[5]	0	[5]	0	[5]
Net (loss) income attributable to Fannie Mae	1,651	[5]		335	[5]	3,679	[5]	1,211	[5]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	369	[6],[7]		485	[6],[7]	1,258	[6],[7]	1,433	[6],[7]
Benefit (provision) for credit losses	0	[6]		0	[6]	0	[6]	0	[6]
Net interest income (loss) after benefit (provision) for credit losses	369	[6]		485	[6]	1,258	[6]	1,433	[6]
Guaranty Fee Income	(1,361)	[1],[6]		(685)	[1],[6]	(3,697)	[1],[6]	(1,918)	[1],[6]
Investment gains (losses), net	(1,023)	[6],[8]		(2,019)	[6],[8]	(2,720)	[6],[8]	(4,366)	[6],[8]
Net other-than-temporary impairments	0	[6]		0	[6]	0	[6]	0	[6]
Fair value gains (losses), net	86	[6],[9]		0	[6],[9]	454	[6],[9]	137	[6],[9]
Debt extinguishment (losses) gains, net	0	[6]		0	[6]	0	[6]	0	[6]
Gains (losses) from partnership investments	7	[6]		(8)	[6]	18	[6]	(4)	[6]
Fee and other income (expense)	72	[6]		(5)	[6]	146	[6]	(16)	[6]
Administrative expenses	0	[6]		0	[6]	0	[6]	0	[6]
Foreclosed property income (expense)	0	[6]		0	[6]	0	[6]	0	[6]
Other (expenses) income	(79)	[6]		(17)	[6]	(118)	[6]	(54)	[6]
Income (loss) before federal income taxes	(1,929)	[6]		(2,249)	[6]	(4,659)	[6]	(4,788)	[6]
Benefit (Provision) for federal income taxes	0	[6]		0	[6]	0	[4],[6]	0	[6]
Net income (loss)	(1,929)	[6]		(2,249)	[6]	(4,659)	[6]	(4,788)	[6]
Less: Net (income) loss attributable to noncontrolling interest	(7)	[10],[6]		8	[10],[6]	(18)	[10],[6]	4	[10],[6]
Net (loss) income attributable to Fannie Mae	$ (1,936)	[6]		$ (2,241)	[6]	$ (4,677)	[6]	$ (4,784)	[6]

[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 condensed consolidated statements of operations and comprehensive income.

[2] Includes $26 million of other-than-temporary impairments recognized in earnings for the three and nine months ended September 30, 2013, as we had the intent to sell the related securities before recovery of their amortized cost basis.

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

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

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

[6] 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.

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

[8] 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 group’s 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.

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

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