Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,588	$ 5,184	$ 16,332	$ 14,826
Benefit for credit losses	1,550	1,085	1,050	3,498
Net interest income (loss) after benefit for credit losses	7,138	6,269	17,382	18,324
Guaranty fee income (expense)	31	52	99	138
Investment gains (losses), net	299	171	1,155	749
Fair value gains (losses), net	(2,589)	(207)	(1,902)	(2,331)
Debt extinguishment gains (losses), net	(11)	11	0	49
Gains (losses) from partnership investments	(5)	52	235	132
Fee and other income (expense)	228	774	1,024	5,426
Administrative expenses	(952)	(706)	(2,364)	(2,075)
Foreclosed property income (expense)	(497)	(249)	(1,152)	227
TCCA Fees	(413)	(351)	(1,192)	(1,008)
Other income (expenses)	(199)	(124)	(647)	(611)
Income (loss) before federal income taxes	3,030	5,692	12,638	19,020
Provision for federal income taxes	(1,070)	(1,787)	(4,150)	(6,123)
Net income	1,960	3,905	8,488	12,897
Less: Net income attributable to noncontrolling interest	0	0	0	(1)
Net income (loss) attributable to Fannie Mae	1,960	3,905	8,488	12,896
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA Fees	(413)	(351)	(1,192)	(1,008)
Single-family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	66	19	93	(24)
Benefit for credit losses	1,545	1,029	967	3,377
Net interest income (loss) after benefit for credit losses	1,611	1,048	1,060	3,353
Guaranty fee income (expense)	3,145	2,945	9,277	8,708
Investment gains (losses), net	(1)	0	(2)	(1)
Fair value gains (losses), net	(4)	(4)	(8)	(11)
Debt extinguishment gains (losses), net	0	0	0	0
Gains (losses) from partnership investments	(12)	0	(27)	0
Fee and other income (expense)	98	146	571	471
Administrative expenses	(649)	(468)	(1,591)	(1,376)
Foreclosed property income (expense)	(516)	(281)	(1,183)	154
Other income (expenses)	(180)	(136)	(669)	(528)
Income (loss) before federal income taxes	3,079	2,899	6,236	9,762
Provision for federal income taxes	(1,040)	(837)	(2,040)	(2,897)
Net income				6,865
Less: Net income attributable to noncontrolling interest				0
Net income (loss) attributable to Fannie Mae	2,039	2,062	4,196	6,865
Single-family [Member] | US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA Fees	(413)	(351)	(1,192)	(1,008)
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	(16)	(22)	(73)	(65)
Benefit for credit losses	5	56	83	121
Net interest income (loss) after benefit for credit losses	(11)	34	10	56
Guaranty fee income (expense)	367	332	1,064	960
Investment gains (losses), net	5	8	29	50
Fair value gains (losses), net	0	0	0	0
Debt extinguishment gains (losses), net	0	0	0	0
Gains (losses) from partnership investments	7	52	262	131
Fee and other income (expense)	58	32	193	87
Administrative expenses	(109)	(77)	(280)	(225)
Foreclosed property income (expense)	19	32	31	73
Other income (expenses)	5	8	(8)	(5)
Income (loss) before federal income taxes	341	421	1,301	1,127
Provision for federal income taxes	(17)	(37)	(128)	(37)
Net income				1,090
Less: Net income attributable to noncontrolling interest				0
Net income (loss) attributable to Fannie Mae	324	384	1,173	1,090
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	1,401	1,845	4,516	5,592
Benefit for credit losses	0	0	0	0
Net interest income (loss) after benefit for credit losses	1,401	1,845	4,516	5,592
Guaranty fee income (expense)	(210)	(235)	(658)	(722)
Investment gains (losses), net	1,608	1,510	4,679	4,420
Fair value gains (losses), net	(2,697)	(335)	(2,112)	(2,770)
Debt extinguishment gains (losses), net	(25)	(3)	(30)	31
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	83	579	288	4,848
Administrative expenses	(194)	(161)	(493)	(474)
Foreclosed property income (expense)	0	0	0	0
Other income (expenses)	24	(5)	18	(70)
Income (loss) before federal income taxes	(10)	3,195	6,208	10,855
Provision for federal income taxes	(13)	(913)	(1,982)	(3,189)
Net income				7,666
Less: Net income attributable to noncontrolling interest				0
Net income (loss) attributable to Fannie Mae	(23)	2,282	4,226	7,666
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	3,901	3,088	11,039	8,525
Benefit for credit losses	0	0	0	0
Net interest income (loss) after benefit for credit losses	3,901	3,088	11,039	8,525
Guaranty fee income (expense)	(2,024)	(1,488)	(5,929)	(4,367)
Investment gains (losses), net	(187)	21	(664)	(141)
Fair value gains (losses), net	32	13	37	232
Debt extinguishment gains (losses), net	14	14	30	18
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	(80)	(71)	(249)	(242)
Administrative expenses	0	0	0	0
Foreclosed property income (expense)	0	0	0	0
Other income (expenses)	(93)	0	(93)	0
Income (loss) before federal income taxes	1,563	1,577	4,171	4,025
Provision for federal income taxes	0	0	0	0
Net income				4,025
Less: Net income attributable to noncontrolling interest				0
Net income (loss) attributable to Fannie Mae	1,563	1,577	4,171	4,025
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	236	254	757	798
Benefit for credit losses	0	0	0	0
Net interest income (loss) after benefit for credit losses	236	254	757	798
Guaranty fee income (expense)	(1,247)	(1,502)	(3,655)	(4,441)
Investment gains (losses), net	(1,126)	(1,368)	(2,887)	(3,579)
Fair value gains (losses), net	80	119	181	218
Debt extinguishment gains (losses), net	0	0	0	0
Gains (losses) from partnership investments	0	0	0	1
Fee and other income (expense)	69	88	221	262
Administrative expenses	0	0	0	0
Foreclosed property income (expense)	0	0	0	0
Other income (expenses)	45	9	105	(8)
Income (loss) before federal income taxes	(1,943)	(2,400)	(5,278)	(6,749)
Provision for federal income taxes	0	0	0	0
Net income				(6,749)
Less: Net income attributable to noncontrolling interest				(1)
Net income (loss) attributable to Fannie Mae	$ (1,943)	$ (2,400)	$ (5,278)	$ (6,750)