Segment Reporting Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,142	$ 5,184	$ 4,904	$ 4,738	$ 4,851	$ 5,582	$ 5,667	$ 6,304	$ 19,968	$ 22,404	$ 21,501
Benefit for credit losses	466	1,085	1,639	774	0	2,609	5,383	957	3,964	8,949	852
Net interest income after benefit for credit losses	5,608	6,269	6,543	5,512	4,851	8,191	11,050	7,261	23,932	31,353	22,353
Guaranty fee income (expense)									175	205	212
Investment (losses) gains, net									936	1,127	(226)
Fair value (losses) gains, net	(2,502)	(207)	(934)	(1,190)	961	335	829	834	(4,833)	2,959	(2,977)
Debt extinguishment gains, net	17	11	38	0	35	92	27	(23)	66	131	(244)
(Losses) gains from partnership investments									269	517	119
Fee and other income (expense)									5,712	3,725	1,275
Administrative expenses	(702)	(706)	(697)	(672)	(632)	(646)	(626)	(641)	(2,777)	(2,545)	(2,367)
Foreclosed property (expense) income	(369)	(249)	214	262	1,082	1,165	332	260	(142)	2,839	254
Other expenses									(813)	(743)	(941)
Income before federal income taxes	2,130	5,692	5,419	7,909	8,274	10,099	12,080	8,114	21,150	38,567	17,220
(Provision) benefit for federal income taxes	(818)	(1,787)	(1,752)	(2,584)	(1,816)	(1,355)	(1,985)	50,571	(6,941)	45,415	0
Net income	1,312	3,905	3,667	5,325	6,458	8,744	10,095	58,685	14,209	83,982	17,220
Less: Net (income) loss attributable to noncontrolling interest	0	0	(1)	0	(1)	(7)	(11)	0	(1)	(19)	4
Net income attributable to Fannie Mae	1,312	3,905	3,666	5,325	6,457	8,737	10,084	58,685	14,208	83,963	17,224
Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									6	205	(790)
Benefit for credit losses									3,850	8,469	672
Net interest income after benefit for credit losses									3,856	8,674	(118)
Guaranty fee income (expense)									11,702	10,468	8,151
Investment (losses) gains, net									(1)	3	8
Fair value (losses) gains, net									(19)	(10)	(8)
Debt extinguishment gains, net									0	0	0
(Losses) gains from partnership investments									(31)	0	0
Fee and other income (expense)									624	630	759
Administrative expenses									(1,830)	(1,706)	(1,590)
Foreclosed property (expense) income									(225)	2,736	247
Other expenses									(726)	(628)	(841)
Income before federal income taxes									11,975	19,166	6,370
(Provision) benefit for federal income taxes									(3,496)	29,110	(80)
Net income									8,479	48,276	6,290
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									8,479	48,276	6,290
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									(79)	(74)	(13)
Benefit for credit losses									114	480	180
Net interest income after benefit for credit losses									35	406	167
Guaranty fee income (expense)									1,297	1,217	1,040
Investment (losses) gains, net									57	21	37
Fair value (losses) gains, net									0	0	0
Debt extinguishment gains, net									0	0	0
(Losses) gains from partnership investments									299	498	123
Fee and other income (expense)									166	182	207
Administrative expenses									(306)	(280)	(269)
Foreclosed property (expense) income									83	103	7
Other expenses									(10)	(2)	(5)
Income before federal income taxes									1,621	2,145	1,307
(Provision) benefit for federal income taxes									(158)	7,924	204
Net income									1,463	10,069	1,511
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									1,463	10,069	1,511
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									7,243	9,764	13,241
Benefit for credit losses									0	0	0
Net interest income after benefit for credit losses									7,243	9,764	13,241
Guaranty fee income (expense)									(955)	(1,115)	(1,291)
Investment (losses) gains, net									6,378	4,847	5,506
Fair value (losses) gains, net									(5,476)	3,148	(3,041)
Debt extinguishment gains, net									35	27	(277)
(Losses) gains from partnership investments									0	0	0
Fee and other income (expense)									4,894	3,010	717
Administrative expenses									(641)	(559)	(508)
Foreclosed property (expense) income									0	0	0
Other expenses									(77)	20	(22)
Income before federal income taxes									11,401	19,142	14,325
(Provision) benefit for federal income taxes									(3,287)	8,381	(124)
Net income									8,114	27,523	14,201
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									8,114	27,523	14,201
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									11,722	10,939	7,156
Benefit for credit losses									0	0	0
Net interest income after benefit for credit losses									11,722	10,939	7,156
Guaranty fee income (expense)									(5,895)	(5,233)	(4,737)
Investment (losses) gains, net									(192)	(122)	(3)
Fair value (losses) gains, net									300	(722)	(313)
Debt extinguishment gains, net									31	104	33
(Losses) gains from partnership investments									0	0	0
Fee and other income (expense)									(323)	(321)	(395)
Administrative expenses									0	0	0
Foreclosed property (expense) income									0	0	0
Other expenses									0	0	0
Income before federal income taxes									5,643	4,645	1,741
(Provision) benefit for federal income taxes									0	0	0
Net income									5,643	4,645	1,741
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									5,643	4,645	1,741
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									1,076	1,570	1,907
Benefit for credit losses									0	0	0
Net interest income after benefit for credit losses									1,076	1,570	1,907
Guaranty fee income (expense)									(5,974)	(5,132)	(2,951)
Investment (losses) gains, net									(5,306)	(3,622)	(5,774)
Fair value (losses) gains, net									362	543	385
Debt extinguishment gains, net									0	0	0
(Losses) gains from partnership investments									1	19	(4)
Fee and other income (expense)									351	224	(13)
Administrative expenses									0	0	0
Foreclosed property (expense) income									0	0	0
Other expenses									0	(133)	(73)
Income before federal income taxes									(9,490)	(6,531)	(6,523)
(Provision) benefit for federal income taxes									0	0	0
Net income									(9,490)	(6,531)	(6,523)
Less: Net (income) loss attributable to noncontrolling interest									(1)	(19)	4
Net income attributable to Fannie Mae									(9,491)	(6,550)	(6,519)
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(367)	(351)	(335)	(322)	(306)	(276)	(233)	(186)	(1,375)	(1,001)	(238)
US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
TCCA fees									$ (1,375)	$ (1,001)	$ (238)