Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 7,086	$ 6,656	$ 5,777	$ 5,347	$ 5,922	$ 5,348	$ 5,227	$ 4,796	$ 24,866	$ 21,293	$ 21,273
Fee and other income									462	566	555
Net revenues									25,328	21,859	21,828
Investment gains, net	263	653	149	(158)	923	253	461	133	907	1,770	952
Fair value gains (losses), net	(880)	(327)	(1,018)	(276)	84	(713)	(754)	(831)	(2,501)	(2,214)	1,121
Administrative expenses	(803)	(762)	(754)	(749)	(786)	(749)	(744)	(744)	(3,068)	(3,023)	(3,059)
Benefit (provision) for credit losses	1,416	501	(12)	(2,583)	279	1,857	1,225	650	(678)	4,011	3,309
Foreclosed property income (expense)					(151)	(96)	(128)	(140)	(177)	(515)	(617)
Credit-related income (expense)									(855)	3,496	2,692
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees	(697)	(679)	(660)	(637)	(626)	(613)	(600)	(593)	(2,673)	(2,432)	(2,284)
Other expenses, net	(339)	(313)	(261)	(218)	(194)	(186)	(203)	(162)	(1,131)	(745)	(472)
Income before federal income taxes	5,709	5,378	3,214	578	5,230	4,999	4,321	3,027	14,879	17,577	20,099
Provision for federal income taxes	(1,139)	(1,149)	(669)	(117)	(865)	(1,036)	(889)	(627)	(3,074)	(3,417)	(4,140)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	4,570	4,229	2,545	461	4,365	3,963	3,432	2,400	11,805	14,160	15,959
Credit Enhancement Expense	(300)	(325)	(360)	(376)	$ (352)	$ (290)	$ (276)	$ (216)	(1,361)	(1,134)	(679)
Change in expected credit enhancement recoveries	$ (180)	$ (48)	$ 273	$ 188					233	0	0
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									(2,673)	(2,432)	(2,284)
Operating Segments [Member] | FNM_MultifamilyMember
Segment Reporting Information [Line Items]
Net interest income									3,364	3,280	3,111
Fee and other income									94	113	105
Net revenues									3,458	3,393	3,216
Investment gains, net									179	181	102
Fair value gains (losses), net									38	2	(89)
Administrative expenses									(509)	(458)	(428)
Benefit (provision) for credit losses									(603)	(27)	(4)
Foreclosed property income (expense)									(20)	8	(13)
Credit-related income (expense)									(623)	(19)	(17)
Other expenses, net									(76)	(11)
Income before federal income taxes									2,391	2,881	2,645
Provision for federal income taxes									(467)	(558)	(432)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									1,924	2,323	2,213
Credit Enhancement Expense									(220)	(207)	(165)
Change in expected credit enhancement recoveries									144	0	0
Other Income											(26)
Operating Segments [Member] | FNM_SingleFamily
Segment Reporting Information [Line Items]
Net interest income									21,502	18,013	18,162
Fee and other income									368	453	450
Net revenues									21,870	18,466	18,612
Investment gains, net									728	1,589	850
Fair value gains (losses), net									(2,539)	(2,216)	1,210
Administrative expenses									(2,559)	(2,565)	(2,631)
Benefit (provision) for credit losses									(75)	4,038	3,313
Foreclosed property income (expense)									(157)	(523)	(604)
Credit-related income (expense)									(232)	3,515	2,709
Other expenses, net									(1,055)	(734)	(498)
Income before federal income taxes									12,488	14,696	17,454
Provision for federal income taxes									(2,607)	(2,859)	(3,708)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest									9,881	11,837	13,746
Credit Enhancement Expense									(1,141)	(927)	(514)
Change in expected credit enhancement recoveries									89	0	0
Operating Segments [Member] | US Treasury [Member] | FNM_MultifamilyMember
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									0	0	0
Operating Segments [Member] | US Treasury [Member] | FNM_SingleFamily
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									$ (2,673)	$ (2,432)	$ (2,284)