Segment Reporting Segment Reporting(Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					6 Months Ended
	Jun. 30, 2013		Mar. 31, 2013	Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 5,667			$ 5,428		$ 11,971		$ 10,625
Benefit (provision) for credit losses	5,383			3,041		6,340		1,041
Net interest income (loss) after benefit (provision) for credit losses	11,050			8,469		18,311		11,666
Guaranty fee income (expense)	51	[1]		58	[1]	105	[1]	120	[1]
Investment gains (losses), net	290			131		408		247
Net other-than-temporary impairments	(6)			(599)		(15)		(663)
Fair value gains (losses), net	829			(2,449)		1,663		(2,166)
Debt extinguishment (losses) gains, net	27			(93)		4		(127)
Gains (losses) from partnership investments	115	[2]		23	[2]	174	[2]	33	[2]
Fee and other income (expense)	434			337		948		650
Administrative expenses	(626)			(567)		(1,267)		(1,131)
Foreclosed property income (expense)	332			70		592		(269)
Other (expenses) income	(416)			(261)		(729)		(523)
Income (loss) before federal income taxes	12,080			5,119		20,194		7,837
Benefit (provision) for federal income taxes	(1,985)		50,600	0		48,586	[3]	0
Net income (loss)	10,095			5,119		68,780		7,837
Less: Net (income) loss attributable to noncontrolling interest	(11)			(5)		(11)		(4)
Net (loss) income attributable to Fannie Mae	10,084			5,114		68,769		7,833
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(50)			(215)		470		(594)
Benefit (provision) for credit losses	5,353			2,956		6,134		903
Net interest income (loss) after benefit (provision) for credit losses	5,303			2,741		6,604		309
Guaranty fee income (expense)	2,544			1,970		4,919		3,881
Investment gains (losses), net	1			2		3		3
Net other-than-temporary impairments	0			0		0		0
Fair value gains (losses), net	(1)			(3)		(3)		(4)
Debt extinguishment (losses) gains, net	0			0		0		0
Gains (losses) from partnership investments	0			0		0		0
Fee and other income (expense)	179			207		351		407
Administrative expenses	(419)			(382)		(845)		(762)
Foreclosed property income (expense)	328			59		581		(273)
Other (expenses) income	(387)			(240)		(741)		(475)
Income (loss) before federal income taxes	7,548					10,869
Benefit (provision) for federal income taxes	(1,050)					30,528	[3]
Net income (loss)	6,498			4,354		41,397		3,086
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	6,498			4,354		41,397		3,086
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(17)			(6)		(28)		(13)
Benefit (provision) for credit losses	30			85		206		138
Net interest income (loss) after benefit (provision) for credit losses	13			79		178		125
Guaranty fee income (expense)	300			252		591		495
Investment gains (losses), net	4			6		11		12
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	104			18		163		29
Fee and other income (expense)	38			49		89		96
Administrative expenses	(67)			(60)		(137)		(124)
Foreclosed property income (expense)	4			11		11		4
Other (expenses) income	0			3		1		0
Income (loss) before federal income taxes	396					907
Benefit (provision) for federal income taxes	(10)					7,978	[3]
Net income (loss)	386			358		8,885		637
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	386			358		8,885		637
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	2,680			3,443		5,422		6,984
Benefit (provision) for credit losses	0			0		0		0
Net interest income (loss) after benefit (provision) for credit losses	2,680			3,443		5,422		6,984
Guaranty fee income (expense)	(283)			(326)		(582)		(658)
Investment gains (losses), net	898			1,458		2,247		2,465
Net other-than-temporary impairments	(6)			(597)		(15)		(661)
Fair value gains (losses), net	841			(2,461)		1,716		(2,291)
Debt extinguishment (losses) gains, net	3			(102)		(37)		(172)
Gains (losses) from partnership investments	0			0		0		0
Fee and other income (expense)	255			186		604		366
Administrative expenses	(140)			(125)		(285)		(245)
Foreclosed property income (expense)	0			0		0		0
Other (expenses) income	(8)			(3)		50		(11)
Income (loss) before federal income taxes	4,240					9,120
Benefit (provision) for federal income taxes	(925)					10,080	[3]
Net income (loss)	3,315			1,473		19,200		5,777
Less: Net (income) loss attributable to noncontrolling interest	0			0		0		0
Net (loss) income attributable to Fannie Mae	3,315			1,473		19,200		5,777
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	2,621	[4]		1,731	[4]	5,218	[4]	3,300	[4]
Benefit (provision) for credit losses	0	[4]		0	[4]	0	[4]	0	[4]
Net interest income (loss) after benefit (provision) for credit losses	2,621	[4]		1,731	[4]	5,218	[4]	3,300	[4]
Guaranty fee income (expense)	(1,283)	[1],[4]		(1,206)	[1],[4]	(2,487)	[1],[4]	(2,365)	[1],[4]
Investment gains (losses), net	(89)	[4]		87	[4]	(156)	[4]	114	[4]
Net other-than-temporary impairments	0	[4]		(2)	[4]	0	[4]	(2)	[4]
Fair value gains (losses), net	(214)	[4]		(60)	[4]	(418)	[4]	(8)	[4]
Debt extinguishment (losses) gains, net	24	[4]		9	[4]	41	[4]	45	[4]
Gains (losses) from partnership investments	0	[4]		0	[4]	0	[4]	0	[4]
Fee and other income (expense)	(86)	[4]		(100)	[4]	(170)	[4]	(208)	[4]
Administrative expenses	0	[4]		0	[4]	0	[4]	0	[4]
Foreclosed property income (expense)	0	[4]		0	[4]	0	[4]	0	[4]
Other (expenses) income	0	[4]		0	[4]	0	[4]	0	[4]
Income (loss) before federal income taxes	973	[4]				2,028	[4]
Benefit (provision) for federal income taxes	0	[4]				0	[3],[4]
Net income (loss)	973	[4]		459	[4]	2,028	[4]	876	[4]
Less: Net (income) loss attributable to noncontrolling interest	0	[4]		0	[4]	0	[4]	0	[4]
Net (loss) income attributable to Fannie Mae	973	[4]		459	[4]	2,028	[4]	876	[4]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	433	[5],[6]		475	[5],[6]	889	[5],[6]	948	[5],[6]
Benefit (provision) for credit losses	0	[6]		0	[6]	0	[6]	0	[6]
Net interest income (loss) after benefit (provision) for credit losses	433	[6]		475	[6]	889	[6]	948	[6]
Guaranty fee income (expense)	(1,227)	[1],[6]		(632)	[1],[6]	(2,336)	[1],[6]	(1,233)	[1],[6]
Investment gains (losses), net	(524)	[6],[7]		(1,422)	[6],[7]	(1,697)	[6],[7]	(2,347)	[6],[7]
Net other-than-temporary impairments	0	[6]		0	[6]	0	[6]	0	[6]
Fair value gains (losses), net	203	[6],[8]		75	[6],[8]	368	[6],[8]	137	[6],[8]
Debt extinguishment (losses) gains, net	0	[6]		0	[6]	0	[6]	0	[6]
Gains (losses) from partnership investments	11	[6]		5	[6]	11	[6]	4	[6]
Fee and other income (expense)	48	[6]		(5)	[6]	74	[6]	(11)	[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	(21)	[6]		(21)	[6]	(39)	[6]	(37)	[6]
Income (loss) before federal income taxes	(1,077)	[6]				(2,730)	[6]
Benefit (provision) for federal income taxes	0	[6]				0	[3],[6]
Net income (loss)	(1,077)	[6]		(1,525)	[6]	(2,730)	[6]	(2,539)	[6]
Less: Net (income) loss attributable to noncontrolling interest	(11)	[6],[9]		(5)	[6],[9]	(11)	[6],[9]	(4)	[6],[9]
Net (loss) income attributable to Fannie Mae	$ (1,088)	[6]		$ (1,530)	[6]	$ (2,741)	[6]	$ (2,543)	[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] Gains from partnership investments are included in other expenses in our condensed consolidated statements of operations and comprehensive income.

[3] 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.

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

[5] 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.

[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] 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.

[8] 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.

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