Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,677	$ 4,904	$ 10,744	$ 9,642
(Provision) benefit for credit losses	(1,033)	1,639	(500)	2,413
Net interest (loss) income after (provision) benefit for credit losses	4,644	6,543	10,244	12,055
Guaranty fee income (expense)	37	41	68	86
Investment gains (losses), net	514	483	856	578
Fair value gains (losses), net	2,606	(934)	687	(2,124)
Debt extinguishment (losses) gains, net	3	38	11	38
Gains (losses) from partnership investments	33	35	240	80
Fee and other income (expense)	519	342	796	4,652
Administrative expenses	(689)	(697)	(1,412)	(1,369)
Foreclosed property (expense) income	(182)	214	(655)	476
TCCA Fees	(397)	(335)	(779)	(657)
Other (expenses) income	(238)	(311)	(448)	(487)
Income before federal income taxes	6,850	5,419	9,608	13,328
Provision for federal income taxes	(2,210)	(1,752)	(3,080)	(4,336)
Net income	4,640	3,667	6,528	8,992
Less: Net income attributable to noncontrolling interest	0	(1)	0	(1)
Net income attributable to Fannie Mae	4,640	3,666	6,528	8,991
Single-family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	18	5	27	(43)
(Provision) benefit for credit losses	(1,056)	1,603	(578)	2,348
Net interest (loss) income after (provision) benefit for credit losses	(1,038)	1,608	(551)	2,305
Guaranty fee income (expense)	3,092	2,893	6,132	5,763
Investment gains (losses), net	(1)	(1)	(1)	(1)
Fair value gains (losses), net	0	(2)	(4)	(7)
Debt extinguishment (losses) gains, net	0	0	0	0
Gains (losses) from partnership investments	(10)	0	(15)	0
Fee and other income (expense)	301	181	473	325
Administrative expenses	(458)	(458)	(942)	(908)
Foreclosed property (expense) income	(182)	178	(667)	435
Other (expenses) income	(262)	(237)	(489)	(392)
Income before federal income taxes	1,045	3,827	3,157	6,863
Provision for federal income taxes	(419)	(1,133)	(1,000)	(2,060)
Net income		2,694		4,803
Less: Net income attributable to noncontrolling interest		0		0
Net income attributable to Fannie Mae	626	2,694	2,157	4,803
Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	(26)	(21)	(57)	(43)
(Provision) benefit for credit losses	23	36	78	65
Net interest (loss) income after (provision) benefit for credit losses	(3)	15	21	22
Guaranty fee income (expense)	357	317	697	628
Investment gains (losses), net	15	39	24	42
Fair value gains (losses), net	0	0	0	0
Debt extinguishment (losses) gains, net	0	0	0	0
Gains (losses) from partnership investments	43	34	255	79
Fee and other income (expense)	84	31	135	55
Administrative expenses	(83)	(75)	(171)	(148)
Foreclosed property (expense) income	0	36	12	41
Other (expenses) income	(6)	(12)	(13)	(13)
Income before federal income taxes	407	385	960	706
Provision for federal income taxes	(41)	(9)	(111)	0
Net income		376		706
Less: Net income attributable to noncontrolling interest		0		0
Net income attributable to Fannie Mae	366	376	849	706
Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	1,513	1,917	3,115	3,747
(Provision) benefit for credit losses	0	0	0	0
Net interest (loss) income after (provision) benefit for credit losses	1,513	1,917	3,115	3,747
Guaranty fee income (expense)	(221)	(241)	(448)	(487)
Investment gains (losses), net	1,562	1,625	3,071	2,910
Fair value gains (losses), net	2,555	(1,098)	585	(2,435)
Debt extinguishment (losses) gains, net	(7)	41	(5)	34
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	150	136	205	4,269
Administrative expenses	(148)	(164)	(299)	(313)
Foreclosed property (expense) income	0	0	0	0
Other (expenses) income	(4)	(57)	(6)	(65)
Income before federal income taxes	5,400	2,159	6,218	7,660
Provision for federal income taxes	(1,750)	(610)	(1,969)	(2,276)
Net income		1,549		5,384
Less: Net income attributable to noncontrolling interest		0		0
Net income attributable to Fannie Mae	3,650	1,549	4,249	5,384
Consolidated Trusts [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	3,922	2,739	7,138	5,437
(Provision) benefit for credit losses	0	0	0	0
Net interest (loss) income after (provision) benefit for credit losses	3,922	2,739	7,138	5,437
Guaranty fee income (expense)	(2,343)	(1,452)	(3,905)	(2,879)
Investment gains (losses), net	(275)	(104)	(477)	(162)
Fair value gains (losses), net	7	171	5	219
Debt extinguishment (losses) gains, net	10	(3)	16	4
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	(90)	(95)	(169)	(171)
Administrative expenses	0	0	0	0
Foreclosed property (expense) income	0	0	0	0
Other (expenses) income	0	0	0	0
Income before federal income taxes	1,231	1,256	2,608	2,448
Provision for federal income taxes	0	0	0	0
Net income		1,256		2,448
Less: Net income attributable to noncontrolling interest		0		0
Net income attributable to Fannie Mae	1,231	1,256	2,608	2,448
Eliminations/Adjustments [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	250	264	521	544
(Provision) benefit for credit losses	0	0	0	0
Net interest (loss) income after (provision) benefit for credit losses	250	264	521	544
Guaranty fee income (expense)	(848)	(1,476)	(2,408)	(2,939)
Investment gains (losses), net	(787)	(1,076)	(1,761)	(2,211)
Fair value gains (losses), net	44	(5)	101	99
Debt extinguishment (losses) gains, net	0	0	0	0
Gains (losses) from partnership investments	0	1	0	1
Fee and other income (expense)	74	89	152	174
Administrative expenses	0	0	0	0
Foreclosed property (expense) income	0	0	0	0
Other (expenses) income	34	(5)	60	(17)
Income before federal income taxes	(1,233)	(2,208)	(3,335)	(4,349)
Provision for federal income taxes	0	0	0	0
Net income		(2,208)		(4,349)
Less: Net income attributable to noncontrolling interest		(1)		(1)
Net income attributable to Fannie Mae	(1,233)	(2,209)	(3,335)	(4,350)
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA Fees	(397)	(335)	(779)	(657)
US Treasury [Member] | Single-family [Member]
Segment Reporting Information [Line Items]
TCCA Fees	$ (397)	$ (335)	$ (779)	$ (657)