Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net interest income	$ 5,805	$ 5,435	$ 5,286	$ 4,769	$ 5,077	$ 5,588	$ 5,677	$ 5,067	$ 21,295	$ 21,409	$ 19,968
Fee and other income									966	1,348	5,887
Net revenues									22,261	22,757	25,855
Investment gains, net	322	467	398	69	181	299	514	342	1,256	1,336	936
Fair value losses, net	3,890	(491)	(1,667)	(2,813)	135	(2,589)	2,606	(1,919)	(1,081)	(1,767)	(4,833)
Administrative expenses	(714)	(661)	(678)	(688)	(686)	(952)	(689)	(723)	(2,741)	(3,050)	(2,777)
Benefit for credit losses	(1,303)	673	1,601	1,184	(255)	1,550	(1,033)	533	2,155	795	3,964
Foreclosed property income (expense)	(137)	(110)	(63)	(334)	(477)	(497)	(182)	(473)	(644)	(1,629)	(142)
Credit-related income (expense)									1,511	(834)	3,822
TCCA fees	(487)	(465)	(453)	(440)	(429)	(413)	(397)	(382)	(1,845)	(1,621)	(1,375)
Other expenses, net	(210)	(300)	(254)	(264)	(201)	(215)	(202)	5	(1,028)	(613)	(478)
Income before federal income taxes	7,580	4,723	4,344	1,686	3,570	3,030	6,850	2,758	18,333	16,208	21,150
Provision for federal income taxes	(2,545)	(1,527)	(1,398)	(550)	(1,103)	(1,070)	(2,210)	(870)	(6,020)	(5,253)	(6,941)
Net income					2,467	1,960	4,640	1,888	12,313	10,955	14,209
Less: Net income attributable to noncontrolling interest					(1)	0	0	0	0	(1)	(1)
Net income attributable to Fannie Mae	$ 5,035	$ 3,196	$ 2,946	$ 1,136	$ 2,466	$ 1,960	$ 4,640	$ 1,888	12,313	10,954	14,208
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees									(1,845)	(1,621)	(1,375)
Operating Segments [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income									19,010	19,301	18,204
Fee and other income									521	636	5,393
Net revenues									19,531	19,937	23,597
Investment gains, net									944	970	675
Fair value losses, net									(1,040)	(1,505)	(4,788)
Administrative expenses									(2,418)	(2,711)	(2,467)
Benefit for credit losses									2,092	688	3,850
Foreclosed property income (expense)									(653)	(1,723)	(225)
Credit-related income (expense)									1,439	(1,035)	3,625
Other expenses, net									(1,012)	(831)	(718)
Income before federal income taxes									15,599	13,204	18,549
Provision for federal income taxes									(5,417)	(4,593)	(6,410)
Net income										8,611	12,139
Less: Net income attributable to noncontrolling interest										0	0
Net income attributable to Fannie Mae									10,182	8,611	12,139
Operating Segments [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income									2,285	2,108	1,764
Fee and other income									445	712	494
Net revenues									2,730	2,820	2,258
Investment gains, net									312	366	261
Fair value losses, net									(41)	(262)	(45)
Administrative expenses									(323)	(339)	(310)
Benefit for credit losses									63	107	114
Foreclosed property income (expense)									9	94	83
Credit-related income (expense)									72	201	197
Other expenses, net									(16)	218	240
Income before federal income taxes									2,734	3,004	2,601
Provision for federal income taxes									(603)	(660)	(531)
Net income										2,344	2,070
Less: Net income attributable to noncontrolling interest										(1)	(1)
Net income attributable to Fannie Mae									2,131	2,343	2,069
Operating Segments [Member] | US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
TCCA fees									(1,845)	(1,621)	(1,375)
Operating Segments [Member] | US Treasury [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
TCCA fees									$ 0	$ 0	$ 0