Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,286	$ 5,677	$ 10,055	$ 10,744
Benefit (provision) for credit losses	1,601	(1,033)	2,785	(500)
Net interest income (loss) after benefit (provision) for credit losses	6,887	4,644	12,840	10,244
Guaranty fee income	30	37	55	68
Investment gains (losses), net	398	514	467	856
Fair value gains (losses), net	(1,667)	2,606	(4,480)	687
Gains (losses) from partnership investments	2	33	3	240
Fee and other income (expense)	144	519	322	796
Administrative expenses	(678)	(689)	(1,366)	(1,412)
Foreclosed property income (expense)	(63)	(182)	(397)	(655)
TCCA fees	(453)	(397)	(893)	(779)
Other income (expenses), net	(256)	(235)	(521)	(437)
Income (loss) before federal income taxes	4,344	6,850	6,030	9,608
Provision for federal income taxes	(1,398)	(2,210)	(1,948)	(3,080)
Net income (loss) attributable to Fannie Mae	2,946	4,640	4,082	6,528
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(453)	(397)	(893)	(779)
Business Segments [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	176	18	319	27
Benefit (provision) for credit losses	1,596	(1,056)	2,759	(578)
Net interest income (loss) after benefit (provision) for credit losses	1,772	(1,038)	3,078	(551)
Guaranty fee income	3,260	3,092	6,482	6,132
Investment gains (losses), net	0	(1)	(1)	(1)
Fair value gains (losses), net	0	0	0	(4)
Gains (losses) from partnership investments	(18)	(10)	(37)	(15)
Fee and other income (expense)	100	301	201	473
Administrative expenses	(496)	(458)	(1,004)	(942)
Foreclosed property income (expense)	(61)	(182)	(396)	(667)
Other income (expenses), net	(361)	(262)	(664)	(489)
Income (loss) before federal income taxes	3,743	1,045	6,766	3,157
Provision for federal income taxes	(1,093)	(419)	(1,736)	(1,000)
Net income (loss) attributable to Fannie Mae	2,650	626	5,030	2,157
Business Segments [Member] | Single-Family [Member] | US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	(453)	(397)	(893)	(779)
Business Segments [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	(15)	(26)	(40)	(57)
Benefit (provision) for credit losses	5	23	26	78
Net interest income (loss) after benefit (provision) for credit losses	(10)	(3)	(14)	21
Guaranty fee income	400	357	785	697
Investment gains (losses), net	11	15	14	24
Fair value gains (losses), net	0	0	0	0
Gains (losses) from partnership investments	20	43	40	255
Fee and other income (expense)	48	84	107	135
Administrative expenses	(86)	(83)	(170)	(171)
Foreclosed property income (expense)	(2)	0	(1)	12
Other income (expenses), net	(2)	(6)	(11)	(13)
Income (loss) before federal income taxes	379	407	750	960
Provision for federal income taxes	(40)	(41)	(78)	(111)
Net income (loss) attributable to Fannie Mae	339	366	672	849
Business Segments [Member] | Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	1,080	1,513	2,172	3,115
Benefit (provision) for credit losses	0	0	0	0
Net interest income (loss) after benefit (provision) for credit losses	1,080	1,513	2,172	3,115
Guaranty fee expense	(192)	(221)	(387)	(448)
Investment gains (losses), net	2,088	1,562	3,503	3,071
Fair value gains (losses), net	(1,730)	2,555	(4,533)	585
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	31	150	52	205
Administrative expenses	(96)	(148)	(192)	(299)
Foreclosed property income (expense)	0	0	0	0
Other income (expenses), net	(30)	(11)	(59)	(11)
Income (loss) before federal income taxes	1,151	5,400	556	6,218
Provision for federal income taxes	(265)	(1,750)	(134)	(1,969)
Net income (loss) attributable to Fannie Mae	886	3,650	422	4,249
Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)	4,045	4,172	7,604	7,659
Benefit (provision) for credit losses	0	0	0	0
Net interest income (loss) after benefit (provision) for credit losses	4,045	4,172	7,604	7,659
Guaranty fee expense	(3,438)	(3,191)	(6,825)	(6,313)
Investment gains (losses), net	(1,701)	(1,062)	(3,049)	(2,238)
Fair value gains (losses), net	63	51	53	106
Gains (losses) from partnership investments	0	0	0	0
Fee and other income (expense)	(35)	(16)	(38)	(17)
Administrative expenses	0	0	0	0
Foreclosed property income (expense)	0	0	0	0
Other income (expenses), net	137	44	213	76
Income (loss) before federal income taxes	(929)	(2)	(2,042)	(727)
Provision for federal income taxes	0	0	0	0
Net income (loss) attributable to Fannie Mae	$ (929)	$ (2)	$ (2,042)	$ (727)