Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 5,184		$ 5,582	[1]	$ 14,826		$ 17,553	[1]
Benefit for credit losses	1,085		2,609	[1]	3,498		8,949	[1]
Net interest income after benefit for credit losses	6,269		8,191	[1]	18,324		26,502	[1]
Guaranty Fee Income (expense)	52	[2],[3]	51	[1],[2],[3]	138	[2],[3]	156	[1],[2],[3]
Investment gains (losses), net	177		648	[1]	829		1,056	[1]
Net other-than-temporary impairments	(6)		(27)	[1]	(80)		(42)	[1]
Fair value (losses) gains, net	(207)		335	[1]	(2,331)		1,998	[1]
Debt extinguishment (losses) gains, net	11		92	[1]	49		96	[1]
Gains from partnership investments	52	[4]	128	[1],[4]	132	[4]	302	[1],[4]
Fee and other income (expense)	774		690	[1]	5,426		1,638	[1]
Administrative expenses	(706)		(646)	[1]	(2,075)		(1,913)	[1]
Foreclosed property (expense) income	(249)		1,165	[1]	227		1,757	[1]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(351)		(276)		(1,008)		(695)
Other expenses (income)	(124)		(252)	[1]	(611)		(562)	[1]
Income before federal income taxes	5,692		10,099	[1]	19,020		30,293	[1]
(Provision) benefit for federal income taxes	(1,787)		(1,355)	[1]	(6,123)		47,231	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest	3,905		8,744	[1]	12,897		77,524	[1]
Net Income Attributable to Noncontrolling Interest	0		(7)	[1]	(1)		(18)	[1]
Net income attributable to Fannie Mae	3,905		8,737	[1]	12,896		77,506	[1]
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	19		(152)	[1]	(24)		318	[1]
Benefit for credit losses	1,029		2,471	[1]	3,377		8,605	[1]
Net interest income after benefit for credit losses	1,048		2,319	[1]	3,353		8,923	[1]
Guaranty Fee Income (expense)	2,945	[2]	2,719	[1],[2]	8,708	[2]	7,638	[1],[2]
Investment gains (losses), net	0		1	[1]	(1)		4	[1]
Net other-than-temporary impairments	0		0	[1]	0		0	[1]
Fair value (losses) gains, net	(4)		(2)	[1]	(11)		(5)	[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)	146		151	[1]	471		502	[1]
Administrative expenses	(468)		(436)	[1]	(1,376)		(1,281)	[1]
Foreclosed property (expense) income	(281)		1,171	[1]	154		1,752	[1]
Other expenses (income)	(136)		(151)	[1]	(528)		(473)	[1]
Income before federal income taxes	2,899		5,496	[1]	9,762		16,365	[1]
(Provision) benefit for federal income taxes	(837)		(751)	[1]	(2,897)		29,777	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest			4,745	[1]	6,865		46,142	[1]
Net Income Attributable to Noncontrolling Interest			0	[1]	0		0	[1]
Net income attributable to Fannie Mae	2,062		4,745	[1]	6,865		46,142	[1]
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(22)		(31)	[1]	(65)		(59)	[1]
Benefit for credit losses	56		138	[1]	121		344	[1]
Net interest income after benefit for credit losses	34		107	[1]	56		285	[1]
Guaranty Fee Income (expense)	332	[2]	311	[1],[2]	960	[2]	902	[1],[2]
Investment gains (losses), net	8		3	[1]	50		14	[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	52	[4]	121	[1],[4]	131	[4]	284	[1],[4]
Fee and other income (expense)	32		26	[1]	87		115	[1]
Administrative expenses	(77)		(73)	[1]	(225)		(210)	[1]
Foreclosed property (expense) income	32		(6)	[1]	73		5	[1]
Other expenses (income)	8		(3)	[1]	(5)		(2)	[1]
Income before federal income taxes	421		486	[1]	1,127		1,393	[1]
(Provision) benefit for federal income taxes	(37)		(8)	[1]	(37)		7,970	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest			478	[1]	1,090		9,363	[1]
Net Income Attributable to Noncontrolling Interest			0	[1]	0		0	[1]
Net income attributable to Fannie Mae	384		478	[1]	1,090		9,363	[1]
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	1,845		2,311	[1]	5,592		7,733	[1]
Benefit for credit losses	0		0	[1]	0		0	[1]
Net interest income after benefit for credit losses	1,845		2,311	[1]	5,592		7,733	[1]
Guaranty Fee Income (expense)	(235)	[2]	(273)	[1],[2]	(722)	[2]	(855)	[1],[2]
Investment gains (losses), net	1,516		1,590	[1]	4,500		3,837	[1]
Net other-than-temporary impairments	(6)		(27)	[1]	(80)		(42)	[1]
Fair value (losses) gains, net	(335)		371	[1]	(2,770)		2,087	[1]
Debt extinguishment (losses) gains, net	(3)		54	[1]	31		17	[1]
Gains from partnership investments	0	[4]	0	[1],[4]	0	[4]	0	[1],[4]
Fee and other income (expense)	579		525	[1]	4,848		1,129	[1]
Administrative expenses	(161)		(137)	[1]	(474)		(422)	[1]
Foreclosed property (expense) income	0		0	[1]	0		0	[1]
Other expenses (income)	(5)		(19)	[1]	(70)		31	[1]
Income before federal income taxes	3,195		4,395	[1]	10,855		13,515	[1]
(Provision) benefit for federal income taxes	(913)		(596)	[1]	(3,189)		9,484	[1],[5]
Net Income, Including Portion Attributable to Noncontrolling Interest			3,799	[1]	7,666		22,999	[1]
Net Income Attributable to Noncontrolling Interest			0	[1]	0		0	[1]
Net income attributable to Fannie Mae	2,282		3,799	[1]	7,666		22,999	[1]
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	3,088	[6]	3,085	[1],[6]	8,525	[6]	8,303	[1],[6]
Benefit for credit losses	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Net interest income after benefit for credit losses	3,088	[6]	3,085	[1],[6]	8,525	[6]	8,303	[1],[6]
Guaranty Fee Income (expense)	(1,488)	[2],[3],[6]	(1,345)	[1],[2],[3],[6]	(4,367)	[2],[3],[6]	(3,832)	[1],[2],[3],[6]
Investment gains (losses), net	21	[6]	77	[1],[6]	(141)	[6]	(79)	[1],[6]
Net other-than-temporary impairments	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Fair value (losses) gains, net	13	[6]	(120)	[1],[6]	232	[6]	(538)	[1],[6]
Debt extinguishment (losses) gains, net	14	[6]	38	[1],[6]	18	[6]	79	[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)	(71)	[6]	(84)	[1],[6]	(242)	[6]	(254)	[1],[6]
Administrative expenses	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Foreclosed property (expense) income	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Other expenses (income)	0	[6]	0	[1],[6]	0	[6]	0	[1],[6]
Income before federal income taxes	1,577	[6]	1,651	[1],[6]	4,025	[6]	3,679	[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			1,651	[1],[6]	4,025	[6]	3,679	[1],[6]
Net Income Attributable to Noncontrolling Interest			0	[1],[6]	0	[6]	0	[1],[6]
Net income attributable to Fannie Mae	1,577	[6]	1,651	[1],[6]	4,025	[6]	3,679	[1],[6]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	254	[7],[8]	369	[1],[7],[8]	798	[7],[8]	1,258	[1],[7],[8]
Benefit for credit losses	0	[8]	0	[1],[8]	0	[8]	0	[1],[8]
Net interest income after benefit for credit losses	254	[8]	369	[1],[8]	798	[8]	1,258	[1],[8]
Guaranty Fee Income (expense)	(1,502)	[2],[3],[8]	(1,361)	[1],[2],[3],[8]	(4,441)	[2],[3],[8]	(3,697)	[1],[2],[3],[8]
Investment gains (losses), net	(1,368)	[8],[9]	(1,023)	[1],[8],[9]	(3,579)	[8],[9]	(2,720)	[1],[8],[9]
Net other-than-temporary impairments	0	[8]	0	[1],[8]	0	[8]	0	[1],[8]
Fair value (losses) gains, net	119	[10],[8]	86	[1],[10],[8]	218	[10],[8]	454	[1],[10],[8]
Debt extinguishment (losses) gains, net	0	[8]	0	[1],[8]	0	[8]	0	[1],[8]
Gains from partnership investments	0	[4],[8]	7	[1],[4],[8]	1	[4],[8]	18	[1],[4],[8]
Fee and other income (expense)	88	[8]	72	[1],[8]	262	[8]	146	[1],[8]
Administrative expenses	0	[8]	0	[1],[8]	0	[8]	0	[1],[8]
Foreclosed property (expense) income	0	[8]	0	[1],[8]	0	[8]	0	[1],[8]
Other expenses (income)	9	[8]	(79)	[1],[8]	(8)	[8]	(118)	[1],[8]
Income before federal income taxes	(2,400)	[8]	(1,929)	[1],[8]	(6,749)	[8]	(4,659)	[1],[8]
(Provision) benefit for federal income taxes	0	[8]	0	[1],[8]	0	[8]	0	[1],[5],[8]
Net Income, Including Portion Attributable to Noncontrolling Interest			(1,929)	[1],[8]	(6,749)	[8]	(4,659)	[1],[8]
Net Income Attributable to Noncontrolling Interest			(7)	[1],[11],[8]	(1)	[11],[8]	(18)	[1],[11],[8]
Net income attributable to Fannie Mae	(2,400)	[8]	(1,936)	[1],[8]	(6,750)	[8]	(4,677)	[1],[8]
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	(351)	[2]	(276)	[1],[2]	(1,008)	[2]	(695)	[1],[2]
US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (TCCA) Fees	$ (351)	[2]	$ (276)	[1],[2]	$ (1,008)	[2]	$ (695)	[1],[2]

[1] Certain prior period amounts have been reclassified to conform to our 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 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.

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