Segment Reporting Segment Reporting(Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 4,904		$ 5,667	[1]	$ 9,642		$ 11,971	[1]
Benefit for credit losses	1,639		5,383	[1]	2,413		6,340	[1]
Net interest income after benefit for credit losses	6,543		11,050	[1]	12,055		18,311	[1]
Guaranty Fee Income (expense)	41	[2],[3]	51	[1],[2],[3]	86	[2],[3]	105	[1],[2],[3]
Investment gains (losses), net	506		290	[1]	652		408	[1]
Net other-than-temporary impairments	(23)		(6)	[1]	(74)		(15)	[1]
Fair value (losses) gains, net	(934)		829	[1]	(2,124)		1,663	[1]
Debt extinguishment (losses) gains, net	38		27	[1]	38		4	[1]
Gains from partnership investments	35	[4]	115	[1],[4]	80	[4]	174	[1],[4]
Fee and other income (expense)	342		434	[1]	4,652		948	[1]
Administrative expenses	(697)		(626)	[1]	(1,369)		(1,267)	[1]
Foreclosed property income	214		332	[1]	476		592	[1]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(335)		(233)		(657)		(419)
Other expenses	(311)		(183)	[1]	(487)		(310)	[1]
Income before federal income taxes	5,419		12,080	[1]	13,328		20,194	[1]
(Provision) benefit for federal income taxes	(1,752)		(1,985)	[1]	(4,336)		48,586	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest	3,667		10,095	[1]	8,992		68,780	[1]
Net Income Attributable to Noncontrolling Interest	(1)		(11)	[1]	(1)		(11)	[1]
Net income attributable to Fannie Mae	3,666		10,084	[1]	8,991		68,769	[1]
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	5		(50)	[1]	(43)		470	[1]
Benefit for credit losses	1,603		5,353	[1]	2,348		6,134	[1]
Net interest income after benefit for credit losses	1,608		5,303	[1]	2,305		6,604	[1]
Guaranty Fee Income (expense)	2,893	[2]	2,544	[1],[2]	5,763	[2]	4,919	[1],[2]
Investment gains (losses), net	(1)		1	[1]	(1)		3	[1]
Net other-than-temporary impairments	0		0	[1]	0		0	[1]
Fair value (losses) gains, net	(2)		(1)	[1]	(7)		(3)	[1]
Debt extinguishment (losses) gains, net	0		0	[1]	0		0	[1]
Gains from partnership investments	0	[4]	0	[1],[4]	0	[4]	0	[1],[4]
Fee and other income (expense)	181		179	[1]	325		351	[1]
Administrative expenses	(458)		(419)	[1]	(908)		(845)	[1]
Foreclosed property income	178		328	[1]	435		581	[1]
Other expenses	(237)		(154)	[1]	(392)		(322)	[1]
Income before federal income taxes	3,827		7,548	[1]	6,863		10,869	[1]
(Provision) benefit for federal income taxes	(1,133)		(1,050)	[1]	(2,060)		30,528	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest	2,694		6,498	[1]	4,803		41,397	[1]
Net Income Attributable to Noncontrolling Interest	0		0	[1]	0		0	[1]
Net income attributable to Fannie Mae	2,694		6,498	[1]	4,803		41,397	[1]
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(21)		(17)	[1]	(43)		(28)	[1]
Benefit for credit losses	36		30	[1]	65		206	[1]
Net interest income after benefit for credit losses	15		13	[1]	22		178	[1]
Guaranty Fee Income (expense)	317	[2]	300	[1],[2]	628	[2]	591	[1],[2]
Investment gains (losses), net	39		4	[1]	42		11	[1]
Net other-than-temporary impairments	0		0	[1]	0		0	[1]
Fair value (losses) gains, net	0		0	[1]	0		0	[1]
Debt extinguishment (losses) gains, net	0		0	[1]	0		0	[1]
Gains from partnership investments	34	[4]	104	[1],[4]	79	[4]	163	[1],[4]
Fee and other income (expense)	31		38	[1]	55		89	[1]
Administrative expenses	(75)		(67)	[1]	(148)		(137)	[1]
Foreclosed property income	36		4	[1]	41		11	[1]
Other expenses	(12)		0	[1]	(13)		1	[1]
Income before federal income taxes	385		396	[1]	706		907	[1]
(Provision) benefit for federal income taxes	(9)		(10)	[1]	0		7,978	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest	376		386	[1]	706		8,885	[1]
Net Income Attributable to Noncontrolling Interest	0		0	[1]	0		0	[1]
Net income attributable to Fannie Mae	376		386	[1]	706		8,885	[1]
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	1,917		2,680	[1]	3,747		5,422	[1]
Benefit for credit losses	0		0	[1]	0		0	[1]
Net interest income after benefit for credit losses	1,917		2,680	[1]	3,747		5,422	[1]
Guaranty Fee Income (expense)	(241)	[2]	(283)	[1],[2]	(487)	[2]	(582)	[1],[2]
Investment gains (losses), net	1,648		898	[1]	2,984		2,247	[1]
Net other-than-temporary impairments	(23)		(6)	[1]	(74)		(15)	[1]
Fair value (losses) gains, net	(1,098)		841	[1]	(2,435)		1,716	[1]
Debt extinguishment (losses) gains, net	41		3	[1]	34		(37)	[1]
Gains from partnership investments	0	[4]	0	[1],[4]	0	[4]	0	[1],[4]
Fee and other income (expense)	136		255	[1]	4,269		604	[1]
Administrative expenses	(164)		(140)	[1]	(313)		(285)	[1]
Foreclosed property income	0		0	[1]	0		0	[1]
Other expenses	(57)		(8)	[1]	(65)		50	[1]
Income before federal income taxes	2,159		4,240	[1]	7,660		9,120	[1]
(Provision) benefit for federal income taxes	(610)		(925)	[1]	(2,276)		10,080	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest	1,549		3,315	[1]	5,384		19,200	[1]
Net Income Attributable to Noncontrolling Interest	0		0	[1]	0		0	[1]
Net income attributable to Fannie Mae	1,549		3,315	[1]	5,384		19,200	[1]
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	2,739	[6]	2,621	[1],[6]	5,437	[6]	5,218	[1],[6]
Benefit for credit losses	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Net interest income after benefit for credit losses	2,739	[6]	2,621	[1],[6]	5,437	[6]	5,218	[1],[6]
Guaranty Fee Income (expense)	(1,452)	[2],[3],[6]	(1,283)	[1],[2],[3],[6]	(2,879)	[2],[3],[6]	(2,487)	[1],[2],[3],[6]
Investment gains (losses), net	(104)	[6]	(89)	[1],[6]	(162)	[6]	(156)	[1],[6]
Net other-than-temporary impairments	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Fair value (losses) gains, net	171	[6]	(214)	[1],[6]	219	[6]	(418)	[1],[6]
Debt extinguishment (losses) gains, net	(3)	[6]	24	[1],[6]	4	[6]	41	[1],[6]
Gains from partnership investments	0	[4],[6]	0	[1],[4],[6]	0	[4],[6]	0	[1],[4],[6]
Fee and other income (expense)	(95)	[6]	(86)	[1],[6]	(171)	[6]	(170)	[1],[6]
Administrative expenses	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Foreclosed property income	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Other expenses	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Income before federal income taxes	1,256	[6]	973	[1],[6]	2,448	[6]	2,028	[1],[6]
(Provision) benefit for federal income taxes	0	[6]	0	[1],[6]	0	[6]	0	[1],[5],[6]
Net Income, Including Portion Attributable to Noncontrolling Interest	1,256	[6]	973	[1],[6]	2,448	[6]	2,028	[1],[6]
Net Income Attributable to Noncontrolling Interest	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Net income attributable to Fannie Mae	1,256	[6]	973	[1],[6]	2,448	[6]	2,028	[1],[6]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	264	[7],[8]	433	[1],[7],[8]	544	[7],[8]	889	[1],[7],[8]
Benefit for credit losses	0	[7]	0	[1],[7]	0	[7]	0	[1],[7]
Net interest income after benefit for credit losses	264	[7]	433	[1],[7]	544	[7]	889	[1],[7]
Guaranty Fee Income (expense)	(1,476)	[2],[3],[7]	(1,227)	[1],[2],[3],[7]	(2,939)	[2],[3],[7]	(2,336)	[1],[2],[3],[7]
Investment gains (losses), net	(1,076)	[7],[9]	(524)	[1],[7],[9]	(2,211)	[7],[9]	(1,697)	[1],[7],[9]
Net other-than-temporary impairments	0	[7]	0	[1],[7]	0	[7]	0	[1],[7]
Fair value (losses) gains, net	(5)	[10],[7]	203	[1],[10],[7]	99	[10],[7]	368	[1],[10],[7]
Debt extinguishment (losses) gains, net	0	[7]	0	[1],[7]	0	[7]	0	[1],[7]
Gains from partnership investments	1	[4],[7]	11	[1],[4],[7]	1	[4],[7]	11	[1],[4],[7]
Fee and other income (expense)	89	[7]	48	[1],[7]	174	[7]	74	[1],[7]
Administrative expenses	0	[7]	0	[1],[7]	0	[7]	0	[1],[7]
Foreclosed property income	0	[7]	0	[1],[7]	0	[7]	0	[1],[7]
Other expenses	(5)	[7]	(21)	[1],[7]	(17)	[7]	(39)	[1],[7]
Income before federal income taxes	(2,208)	[7]	(1,077)	[1],[7]	(4,349)	[7]	(2,730)	[1],[7]
(Provision) benefit for federal income taxes	0	[7]	0	[1],[7]	0	[7]	0	[1],[5],[7]
Net Income, Including Portion Attributable to Noncontrolling Interest	(2,208)	[7]	(1,077)	[1],[7]	(4,349)	[7]	(2,730)	[1],[7]
Net Income Attributable to Noncontrolling Interest	(1)	[11],[7]	(11)	[1],[11],[7]	(1)	[11],[7]	(11)	[1],[11],[7]
Net income attributable to Fannie Mae	(2,209)	[7]	(1,088)	[1],[7]	(4,350)	[7]	(2,741)	[1],[7]
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(335)	[2]	(233)	[1],[2]	(657)	[2]	(419)	[1],[2]
US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	$ (335)	[2]	$ (233)	[1],[2]	$ (657)	[2]	$ (419)	[1],[2]

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

[2] Includes the impact of a 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] Primarily 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 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

[8] 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

[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.

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