Segment Reporting Segment Reporting(Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 4,738		$ 6,304	[1]
Benefit for credit losses	774		957	[1]
Net interest income after benefit for credit losses	5,512		7,261	[1]
Guaranty Fee Income (expense)	45	[2],[3]	54	[1],[2],[3]
Investment gains (losses), net	146		118	[1]
Net other-than-temporary impairments	(51)		(9)	[1]
Fair value (losses) gains, net	(1,190)		834	[1]
Debt extinguishment (losses) gains, net	0		(23)	[1]
Gains from partnership investments	45	[4]	59	[1],[4]
Fee and other income (expense)	4,310		514	[1]
Administrative expenses	(672)		(641)	[1]
Foreclosed property income	262		260	[1]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(322)		(186)
Other expenses	(176)		(127)	[1]
Income before federal income taxes	7,909		8,114	[1]
(Provision) benefit for federal income taxes	(2,584)		50,571	[1],[5]
Net income attributable to Fannie Mae	5,325		58,685	[1]
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(48)		520	[1]
Benefit for credit losses	745		781	[1]
Net interest income after benefit for credit losses	697		1,301	[1]
Guaranty Fee Income (expense)	2,870	[2]	2,375	[1],[2]
Investment gains (losses), net	0		2	[1]
Net other-than-temporary impairments	0		0	[1]
Fair value (losses) gains, net	(5)		(2)	[1]
Debt extinguishment (losses) gains, net	0		0	[1]
Gains from partnership investments	0	[4]	0	[1],[4]
Fee and other income (expense)	144		172	[1]
Administrative expenses	(450)		(426)	[1]
Foreclosed property income	257		253	[1]
Other expenses	(155)		(168)	[1]
Income before federal income taxes	3,036		3,321	[1]
(Provision) benefit for federal income taxes	(927)		31,578	[1],[5]
Net income attributable to Fannie Mae	2,109		34,899	[1]
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(22)		(11)	[1]
Benefit for credit losses	29		176	[1]
Net interest income after benefit for credit losses	7		165	[1]
Guaranty Fee Income (expense)	311	[2]	291	[1],[2]
Investment gains (losses), net	3		7	[1]
Net other-than-temporary impairments	0		0	[1]
Fair value (losses) gains, net	0		0	[1]
Debt extinguishment (losses) gains, net	0		0	[1]
Gains from partnership investments	45	[4]	59	[1],[4]
Fee and other income (expense)	24		51	[1]
Administrative expenses	(73)		(70)	[1]
Foreclosed property income	5		7	[1]
Other expenses	(1)		1	[1]
Income before federal income taxes	321		511	[1]
(Provision) benefit for federal income taxes	9		7,988	[1],[5]
Net income attributable to Fannie Mae	330		8,499	[1]
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	1,830		2,742	[1]
Benefit for credit losses	0		0	[1]
Net interest income after benefit for credit losses	1,830		2,742	[1]
Guaranty Fee Income (expense)	(246)	[2]	(299)	[1],[2]
Investment gains (losses), net	1,336		1,349	[1]
Net other-than-temporary impairments	(51)		(9)	[1]
Fair value (losses) gains, net	(1,337)		875	[1]
Debt extinguishment (losses) gains, net	(7)		(40)	[1]
Gains from partnership investments	0	[4]	0	[1],[4]
Fee and other income (expense)	4,133		349	[1]
Administrative expenses	(149)		(145)	[1]
Foreclosed property income	0		0	[1]
Other expenses	(8)		58	[1]
Income before federal income taxes	5,501		4,880	[1]
(Provision) benefit for federal income taxes	(1,666)		11,005	[1],[5]
Net income attributable to Fannie Mae	3,835		15,885	[1]
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	2,698	[6]	2,597	[1],[6]
Benefit for credit losses	0	[6]	0	[1],[6]
Net interest income after benefit for credit losses	2,698	[6]	2,597	[1],[6]
Guaranty Fee Income (expense)	(1,427)	[2],[3],[6]	(1,204)	[1],[2],[3],[6]
Investment gains (losses), net	(58)	[6]	(67)	[1],[6]
Net other-than-temporary impairments	0	[6]	0	[1],[6]
Fair value (losses) gains, net	48	[6]	(204)	[1],[6]
Debt extinguishment (losses) gains, net	7	[6]	17	[1],[6]
Gains from partnership investments	0	[4],[6]	0	[1],[4],[6]
Fee and other income (expense)	(76)	[6]	(84)	[1],[6]
Administrative expenses	0	[6]	0	[1],[6]
Foreclosed property income	0	[6]	0	[1],[6]
Other expenses	0	[6]	0	[1],[6]
Income before federal income taxes	1,192	[6]	1,055	[1],[6]
(Provision) benefit for federal income taxes	0	[6]	0	[1],[5],[6]
Net income attributable to Fannie Mae	1,192	[6]	1,055	[1],[6]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	280	[7],[8]	456	[1],[7],[8]
Benefit for credit losses	0	[8]	0	[1],[8]
Net interest income after benefit for credit losses	280	[8]	456	[1],[8]
Guaranty Fee Income (expense)	(1,463)	[2],[3],[8]	(1,109)	[1],[2],[3],[8]
Investment gains (losses), net	(1,135)	[8],[9]	(1,173)	[1],[8],[9]
Net other-than-temporary impairments	0	[8]	0	[1],[8]
Fair value (losses) gains, net	104	[10],[8]	165	[1],[10],[8]
Debt extinguishment (losses) gains, net	0	[8]	0	[1],[8]
Gains from partnership investments	0	[4],[8]	0	[1],[8]
Fee and other income (expense)	85	[8]	26	[1],[8]
Administrative expenses	0	[8]	0	[1],[8]
Foreclosed property income	0	[8]	0	[1],[8]
Other expenses	(12)	[8]	(18)	[1],[8]
Income before federal income taxes	(2,141)	[8]	(1,653)	[1],[8]
(Provision) benefit for federal income taxes	0	[8]	0	[1],[5],[8]
Net income attributable to Fannie Mae	(2,141)	[8]	(1,653)	[1],[8]
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(322)	[2]	(186)	[1],[2]
US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	$ (322)	[2]	$ (186)	[1],[2]

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

[2] Includes the impact of the 10 basis point guaranty fee increase implemented pursuant to the TCCA, the incremental revenue from which must be remitted to Treasury. The resulting revenue is included in guaranty fee income and the expense is recognized as “TCCA fees.”

[3] 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.

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

[5] Represented the release of the valuation allowance for our deferred tax assets that generally is directly attributable to each segment based on the nature of the item.

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

[7] 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.

[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] 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.

[10] 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.