Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 5,077	$ 5,588	$ 5,677	$ 5,067	$ 5,142	$ 5,184	$ 4,904	$ 4,738	$ 21,409	$ 19,968	$ 22,404
Benefit for credit losses	(255)	1,550	(1,033)	533	466	1,085	1,639	774	795	3,964	8,949
Net interest income after benefit for credit losses	4,822	7,138	4,644	5,600	5,608	6,269	6,543	5,512	22,204	23,932	31,353
Guaranty fee income (expense)									128	175	205
Investment gains (losses), net	181	299	514	342	187	171	483	95	1,336	936	1,127
Fair value gains (losses), net	135	(2,589)	2,606	(1,919)	(2,502)	(207)	(934)	(1,190)	(1,767)	(4,833)	2,959
Debt extinguishment gains, net									8	66	131
Gains (losses) from partnership investments									244	269	517
Fee and other income (expense)									1,220	5,712	3,725
Administrative expenses	(686)	(952)	(689)	(723)	(702)	(706)	(697)	(672)	(3,050)	(2,777)	(2,545)
Foreclosed property income (expense)	(477)	(497)	(182)	(473)	(369)	(249)	214	262	(1,629)	(142)	2,839
TCCA fees	(429)	(413)	(397)	(382)	(367)	(351)	(335)	(322)	(1,621)	(1,375)	(1,001)
Other expenses									(865)	(813)	(743)
Income before federal income taxes	3,570	3,030	6,850	2,758	2,130	5,692	5,419	7,909	16,208	21,150	38,567
Benefit (provision) for federal income taxes	(1,103)	(1,070)	(2,210)	(870)	(818)	(1,787)	(1,752)	(2,584)	(5,253)	(6,941)	45,415
Net income	2,467	1,960	4,640	1,888	1,312	3,905	3,667	5,325	10,955	14,209	83,982
Less: Net (income) loss attributable to noncontrolling interest	(1)	0	0	0	0	0	(1)	0	(1)	(1)	(19)
Net income attributable to Fannie Mae	2,466	$ 1,960	$ 4,640	$ 1,888	$ 1,312	$ 3,905	$ 3,666	$ 5,325	10,954	14,208	83,963
US Treasury [Member]
Segment Reporting Information [Line Items]
TCCA fees	$ (429)								(1,621)	(1,375)	(1,001)
Operating Segments [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									184	6	205
Benefit for credit losses									688	3,850	8,469
Net interest income after benefit for credit losses									872	3,856	8,674
Guaranty fee income (expense)									12,476	11,702	10,468
Investment gains (losses), net									(2)	(1)	3
Fair value gains (losses), net									(11)	(19)	(10)
Debt extinguishment gains, net									0	0	0
Gains (losses) from partnership investments									(39)	(31)	0
Fee and other income (expense)									666	624	630
Administrative expenses									(2,053)	(1,830)	(1,706)
Foreclosed property income (expense)									(1,723)	(225)	2,736
Other expenses									(942)	(726)	(628)
Income before federal income taxes									7,623	11,975	19,166
Benefit (provision) for federal income taxes									(2,491)	(3,496)	29,110
Net income									5,132	8,479	48,276
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									5,132	8,479	48,276
Operating Segments [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									(92)	(79)	(74)
Benefit for credit losses									107	114	480
Net interest income after benefit for credit losses									15	35	406
Guaranty fee income (expense)									1,439	1,297	1,217
Investment gains (losses), net									33	57	21
Fair value gains (losses), net									0	0	0
Debt extinguishment gains, net									0	0	0
Gains (losses) from partnership investments									282	299	498
Fee and other income (expense)									265	166	182
Administrative expenses									(361)	(306)	(280)
Foreclosed property income (expense)									94	83	103
Other expenses									(13)	(10)	(2)
Income before federal income taxes									1,754	1,621	2,145
Benefit (provision) for federal income taxes									(247)	(158)	7,924
Net income									1,507	1,463	10,069
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									1,507	1,463	10,069
Operating Segments [Member] | Capital Markets [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									5,828	7,243	9,764
Benefit for credit losses									0	0	0
Net interest income after benefit for credit losses									5,828	7,243	9,764
Guaranty fee expense									(863)	(955)	(1,115)
Investment gains (losses), net									5,539	6,378	4,847
Fair value gains (losses), net									(2,049)	(5,476)	3,148
Debt extinguishment gains, net									(37)	35	27
Gains (losses) from partnership investments									0	0	0
Fee and other income (expense)									209	4,894	3,010
Administrative expenses									(636)	(641)	(559)
Foreclosed property income (expense)									0	0	0
Other expenses									8	(77)	20
Income before federal income taxes									7,999	11,401	19,142
Benefit (provision) for federal income taxes									(2,515)	(3,287)	8,381
Net income									5,484	8,114	27,523
Less: Net (income) loss attributable to noncontrolling interest									0	0	0
Net income attributable to Fannie Mae									5,484	8,114	27,523
Operating Segments [Member] | US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
TCCA fees									(1,621)	(1,375)	(1,001)
Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									15,489	12,798	12,509
Benefit for credit losses									0	0	0
Net interest income after benefit for credit losses									15,489	12,798	12,509
Guaranty fee expense									(12,924)	(11,869)	(10,365)
Investment gains (losses), net									(4,234)	(5,498)	(3,744)
Fair value gains (losses), net									293	662	(179)
Debt extinguishment gains, net									45	31	104
Gains (losses) from partnership investments									1	1	19
Fee and other income (expense)									80	28	(97)
Administrative expenses									0	0	0
Foreclosed property income (expense)									0	0	0
Other expenses									82	0	(133)
Income before federal income taxes									(1,168)	(3,847)	(1,886)
Benefit (provision) for federal income taxes									0	0	0
Net income									(1,168)	(3,847)	(1,886)
Less: Net (income) loss attributable to noncontrolling interest									(1)	(1)	(19)
Net income attributable to Fannie Mae									$ (1,169)	$ (3,848)	$ (1,905)