Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net interest income	$ 5,111	$ 5,274	$ 5,002	$ 5,346	$ 5,805	$ 5,435	$ 5,286	$ 4,769	$ 20,733	$ 21,295	$ 21,409
Fee and other income									2,227	966	1,348
Net revenues									22,960	22,261	22,757
Investment gains, net	833	313	385	(9)	322	467	398	69	1,522	1,256	1,336
Fair value losses, net	(191)	(289)	(691)	(40)	3,890	(491)	(1,667)	(2,813)	(1,211)	(1,081)	(1,767)
Administrative expenses	(703)	(664)	(686)	(684)	(714)	(661)	(678)	(688)	(2,737)	(2,741)	(3,050)
Benefit for credit losses	560	(182)	1,267	396	(1,303)	673	1,601	1,184	2,041	2,155	795
Foreclosed property income (expense)	(130)	(140)	(34)	(217)	(137)	(110)	(63)	(334)	(521)	(644)	(1,629)
Credit-related income (expense)									1,520	1,511	(834)
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees	(544)	(531)	(518)	(503)	(487)	(465)	(453)	(440)	(2,096)	(1,845)	(1,621)
Other expenses, net	(411)	(427)	(291)	(382)	(210)	(300)	(254)	(264)	(1,511)	(1,028)	(613)
Income before federal income taxes	4,956	4,548	4,787	4,156	7,580	4,723	4,344	1,686	18,447	18,333	16,208
Provision for federal income taxes	(11,489)	(1,525)	(1,587)	(1,383)	(2,545)	(1,527)	(1,398)	(550)	(15,984)	(6,020)	(5,253)
Net income											10,955
Less: Net income attributable to noncontrolling interest									0	0	(1)
Net income attributable to Fannie Mae	$ (6,533)	$ 3,023	$ 3,200	$ 2,773	$ 5,035	$ 3,196	$ 2,946	$ 1,136	2,463	12,313	10,954
US Treasury [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									(2,096)	(1,845)	(1,621)
Operating Segments [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Net interest income									18,212	19,010	19,301
Fee and other income									1,378	521	636
Net revenues									19,590	19,531	19,937
Investment gains, net									1,352	944	970
Fair value losses, net									(1,188)	(1,040)	(1,505)
Administrative expenses									(2,391)	(2,418)	(2,711)
Benefit for credit losses									2,090	2,092	688
Foreclosed property income (expense)									(540)	(653)	(1,723)
Credit-related income (expense)									1,550	1,439	(1,035)
Other expenses, net									(1,004)	(1,012)	(831)
Income before federal income taxes									15,813	15,599	13,204
Provision for federal income taxes									(14,301)	(5,417)	(4,593)
Net income											8,611
Less: Net income attributable to noncontrolling interest											0
Net income attributable to Fannie Mae									1,512	10,182	8,611
Operating Segments [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Net interest income									2,521	2,285	2,108
Fee and other income									849	445	712
Net revenues									3,370	2,730	2,820
Investment gains, net									170	312	366
Fair value losses, net									(23)	(41)	(262)
Administrative expenses									(346)	(323)	(339)
Benefit for credit losses									(49)	63	107
Foreclosed property income (expense)									19	9	94
Credit-related income (expense)									(30)	72	201
Other expenses, net									(507)	(16)	218
Income before federal income taxes									2,634	2,734	3,004
Provision for federal income taxes									(1,683)	(603)	(660)
Net income											2,344
Less: Net income attributable to noncontrolling interest											(1)
Net income attributable to Fannie Mae									951	2,131	2,343
Operating Segments [Member] | US Treasury [Member] | Single-Family [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									(2,096)	(1,845)	(1,621)
Operating Segments [Member] | US Treasury [Member] | Multifamily [Member]
Segment Reporting Information [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees									$ 0	$ 0	$ 0