Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Information [Line Items]
Net interest (loss) income	$ 5,317		$ 5,186		$ 15,942		$ 15,118
Benefit (provision) for credit losses	(2,079)		(4,151)	[1]	(1,038)		(21,242)	[1]
Net interest income (loss) after benefit (provision) for credit losses	3,238		1,035		14,904		(6,124)
Guaranty fee income (expense)	56	[2]	49	[2]	176	[2]	149	[2]
Investment gains (losses), net	134		73		381		319
Net other-than-temporary impairments	(38)		(262)		(701)		(362)
Fair value gains (losses), net	(1,020)		(4,525)		(3,186)		(5,870)
Debt extinguishment (losses) gains, net	(54)		(119)		(181)		(149)
Gains (losses) from partnership investments	35	[3]	(30)	[3]	68	[3]	(7)	[3]
Fee and other income (expense)	322		242		972		644
Administrative expenses	(588)		(591)		(1,719)		(1,765)
Foreclosed property income (expense)	48		(733)		(221)		(743)
Other (expenses) income	(320)		(224)		(843)		(631)
Income (loss) before federal income taxes	1,813		(5,085)		9,650		(14,539)
Benefit (provision) for federal income taxes	0		0		0		91
Net income (loss)	1,813		(5,085)		9,650		(14,448)
Less: Net (income) loss attributable to the noncontrolling interest	8		0		4		(1)
Net (loss) income attributable to Fannie Mae	1,821		(5,085)		9,654		(14,449)
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(192)		(374)		(786)		(1,952)
Benefit (provision) for credit losses	(2,176)		(4,072)	[1]	(1,273)		(21,104)	[1]
Net interest income (loss) after benefit (provision) for credit losses	(2,368)		(4,446)		(2,059)		(23,056)
Guaranty fee income (expense)	2,014		1,867		5,895		5,618
Investment gains (losses), net	5		3		8		(2)
Net other-than-temporary impairments	0		0		0		0
Fair value gains (losses), net	(1)		(2)		(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)	181		136		588		397
Administrative expenses	(397)		(409)		(1,159)		(1,225)
Foreclosed property income (expense)	46		(710)		(227)		(717)
Other (expenses) income	(290)		(184)		(765)		(579)
Income (loss) before federal income taxes	(810)		(3,745)		2,276		(19,569)
Benefit (provision) for federal income taxes	(12)		(1)		(12)		106
Net income (loss)	(822)				2,264		(19,463)
Less: Net (income) loss attributable to the noncontrolling interest	0				0		0
Net (loss) income attributable to Fannie Mae	(822)		(3,746)		2,264		(19,463)
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	(1)		(7)		(14)		(27)
Benefit (provision) for credit losses	97		(79)	[1]	235		(138)	[1]
Net interest income (loss) after benefit (provision) for credit losses	96		(86)		221		(165)
Guaranty fee income (expense)	265		226		760		651
Investment gains (losses), net	11		5		23		10
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	43		(30)		72		(8)
Fee and other income (expense)	55		51		151		166
Administrative expenses	(70)		(62)		(194)		(194)
Foreclosed property income (expense)	2		(23)		6		(26)
Other (expenses) income	(7)		(9)		(7)		33
Income (loss) before federal income taxes	395		72		1,032		467
Benefit (provision) for federal income taxes	32		0		32		(61)
Net income (loss)	427				1,064		406
Less: Net (income) loss attributable to the noncontrolling interest	0				0		0
Net (loss) income attributable to Fannie Mae	427		72		1,064		406
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	3,247		3,904		10,231		11,481
Benefit (provision) for credit losses	0		0	[1]	0		0	[1]
Net interest income (loss) after benefit (provision) for credit losses	3,247		3,904		10,231		11,481
Guaranty fee income (expense)	(319)		(369)		(977)		(1,159)
Investment gains (losses), net	2,201		801		4,666		2,589
Net other-than-temporary impairments	(38)		(262)		(699)		(361)
Fair value gains (losses), net	(961)		(4,670)		(3,252)		(5,959)
Debt extinguishment (losses) gains, net	(46)		(107)		(218)		(186)
Gains (losses) from partnership investments	0		0		0		0
Fee and other income (expense)	185		125		551		309
Administrative expenses	(121)		(120)		(366)		(346)
Foreclosed property income (expense)	0		0		0		0
Other (expenses) income	(6)		(14)		(17)		(32)
Income (loss) before federal income taxes	4,142		(712)		9,919		6,336
Benefit (provision) for federal income taxes	(20)		1		(20)		46
Net income (loss)	4,122				9,899		6,382
Less: Net (income) loss attributable to the noncontrolling interest	0				0		0
Net (loss) income attributable to Fannie Mae	4,122		(711)		9,899		6,382
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	1,778	[4]	1,210	[4]	5,078	[4]	4,098	[4]
Benefit (provision) for credit losses	0	[4]	0	[1],[4]	0	[4]	0	[1],[4]
Net interest income (loss) after benefit (provision) for credit losses	1,778	[4]	1,210	[4]	5,078	[4]	4,098	[4]
Guaranty fee income (expense)	(1,219)	[2],[4]	(1,124)	[2],[4]	(3,584)	[2],[4]	(3,350)	[2],[4]
Investment gains (losses), net	(64)	[4]	(89)	[4]	50	[4]	(258)	[4]
Net other-than-temporary impairments	0	[4]	0	[4]	(2)	[4]	(1)	[4]
Fair value gains (losses), net	(58)	[4]	(17)	[4]	(66)	[4]	(122)	[4]
Debt extinguishment (losses) gains, net	(8)	[4]	(12)	[4]	37	[4]	37	[4]
Gains (losses) from partnership investments	0	[4]	0	[4]	0	[4]	0	[4]
Fee and other income (expense)	(94)	[4]	(67)	[4]	(302)	[4]	(222)	[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	335	[4]	(99)	[4]	1,211	[4]	182	[4]
Benefit (provision) for federal income taxes	0	[4]	0	[4]	0	[4]	0	[4]
Net income (loss)	335	[4]			1,211	[4]	182	[4]
Less: Net (income) loss attributable to the noncontrolling interest	0	[4]			0	[4]	0	[4]
Net (loss) income attributable to Fannie Mae	335	[4]	(99)	[4]	1,211	[4]	182	[4]
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest (loss) income	485	[5],[6]	453	[5],[6]	1,433	[5],[6]	1,518	[5],[6]
Benefit (provision) for credit losses	0	[5]	0	[1],[5]	0	[5]	0	[1],[5]
Net interest income (loss) after benefit (provision) for credit losses	485	[5]	453	[5]	1,433	[5]	1,518	[5]
Guaranty fee income (expense)	(685)	[2],[5]	(551)	[2],[5]	(1,918)	[2],[5]	(1,611)	[2],[5]
Investment gains (losses), net	(2,019)	[5],[7]	(647)	[5],[7]	(4,366)	[5],[7]	(2,020)	[5],[7]
Net other-than-temporary impairments	0	[5]	0	[5]	0	[5]	0	[5]
Fair value gains (losses), net	0	[5],[8]	164	[5],[8]	137	[5],[8]	216	[5],[8]
Debt extinguishment (losses) gains, net	0	[5]	0	[5]	0	[5]	0	[5]
Gains (losses) from partnership investments	(8)	[5]	0	[5]	(4)	[5]	1	[5]
Fee and other income (expense)	(5)	[5]	(3)	[5]	(16)	[5]	(6)	[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	(17)	[5]	(17)	[5]	(54)	[5]	(53)	[5]
Income (loss) before federal income taxes	(2,249)	[5]	(601)	[5]	(4,788)	[5]	(1,955)	[5]
Benefit (provision) for federal income taxes	0	[5]	0	[5]	0	[5]	0	[5]
Net income (loss)	(2,249)	[5]			(4,788)	[5]	(1,955)	[5]
Less: Net (income) loss attributable to the noncontrolling interest	8	[5],[9]			4	[5],[9]	(1)	[5],[9]
Net (loss) income attributable to Fannie Mae	$ (2,241)	[5]	$ (601)	[5]	$ (4,784)	[5]	$ (1,956)	[5]

[1] Prior period amounts have been reclassified to conform to the current period presentation.

[2] 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 is included in fee and other income in our condensed consolidated statements of operations and comprehensive income (loss).

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

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

[5] 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.

[6] Represents the amortization expense of cost basis adjustments on securities that we own in our portfolio that on a GAAP basis are eliminated.

[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 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 portfolio.

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