Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Interest income:
Trading securities	$ 193	$ 195	$ 176	$ 142	$ 128	$ 140	$ 128	$ 120	$ 706	$ 516	$ 444
Available-for-sale securities	66	77	91	101	113	134	170	203	335	620	1,156
Mortgage loans interest income	27,214	27,047	27,011	27,047	25,814	25,611	26,256	26,961	108,319	104,642	107,699
Other	145	142	115	94	83	66	46	48	496	243	143
Total interest income	27,618	27,461	27,393	27,384	26,138	25,951	26,600	27,332	109,856	106,021	109,442
Interest expense:
Short-term debt interest expense	(77)	(72)	(57)	(44)	(42)	(56)	(57)	(51)	(250)	(206)	(146)
Long-term debt interest expense	(22,430)	(22,115)	(22,334)	(21,994)	(20,291)	(20,460)	(21,257)	(22,512)	(88,873)	(84,520)	(87,887)
Total interest expense	(22,507)	(22,187)	(22,391)	(22,038)	(20,333)	(20,516)	(21,314)	(22,563)	(89,123)	(84,726)	(88,033)
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
Benefit (provision) for credit losses	560	(182)	1,267	396	(1,303)	673	1,601	1,184	2,041	2,155	795
Net interest income after benefit (provision) for credit losses	5,671	5,092	6,269	5,742	4,502	6,108	6,887	5,953	22,774	23,450	22,204
Investment gains (losses), net	833	313	385	(9)	322	467	398	69	1,522	1,256	1,336
Fair value gains (losses), net	(191)	(289)	(691)	(40)	3,890	(491)	(1,667)	(2,813)	(1,211)	(1,081)	(1,767)
Fee and other income	431	1,194	353	249	414	175	174	203	2,227	966	1,348
Non-interest income (loss)	1,073	1,218	47	200	4,626	151	(1,095)	(2,541)	2,538	1,141	917
Administrative expenses:
Salaries and employee benefits	(321)	(331)	(332)	(344)	(319)	(322)	(331)	(364)	(1,328)	(1,336)	(1,319)
Professional services	(252)	(218)	(234)	(229)	(271)	(237)	(232)	(215)	(933)	(955)	(984)
Other administrative expenses	(130)	(115)	(120)	(111)	(124)	(102)	(115)	(109)	(476)	(450)	(747)
Total administrative expenses	(703)	(664)	(686)	(684)	(714)	(661)	(678)	(688)	(2,737)	(2,741)	(3,050)
Foreclosed property expense	(130)	(140)	(34)	(217)	(137)	(110)	(63)	(334)	(521)	(644)	(1,629)
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)
Total expenses	(1,788)	(1,762)	(1,529)	(1,786)	(1,548)	(1,536)	(1,448)	(1,726)	(6,865)	(6,258)	(6,913)
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 (loss) 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
Dividends distributed or available for distribution to senior preferred stockholder	0	(3,048)	(3,117)	(2,779)	(5,471)	(2,977)	(2,869)	(919)	(8,944)	(12,236)	(11,216)
Net income (loss) attributable to common stockholders	$ (6,533)	$ (25)	$ 83	$ (6)	$ (436)	$ 219	$ 77	$ 217	$ (6,481)	$ 77	$ (262)
Earnings (loss) per share: Basic	$ (1.13)	$ 0.00	$ 0.01	$ 0.00	$ (0.08)	$ 0.04	$ 0.01	$ 0.04	$ (1.12)	$ 0.01	$ (0.05)
Earnings (loss) per share: Diluted	$ (1.13)	$ 0.00	$ 0.01	$ 0.00	$ (0.08)	$ 0.04	$ 0.01	$ 0.04	$ (1.12)	$ 0.01	$ (0.05)
Weighted-average common shares outstanding: Basic	5,762	5,762	5,762	5,762	5,762	5,762	5,762	5,762
Weighted-average common shares outstanding: Diluted	5,762	5,762	5,893	5,762	5,762	5,893	5,893	5,893	5,762	5,893	5,762