Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest income:
Trading securities	$ 419	$ 363	$ 318	$ 236	$ 193	$ 195	$ 176	$ 142	$ 1,336	$ 706	$ 516
Available-for-sale securities	55	54	50	71	66	77	91	101	230	335	620
Mortgage loans interest income	29,541	28,723	28,307	28,034	27,214	27,047	27,011	27,047	114,605	108,319	104,642
Interest Income, Federal Funds Sold and Securities Purchased under Agreements to Resell	285	166	149	142	111	109	87	66	742	373	141
Other	34	38	33	31	34	33	28	28	136	123	102
Total interest income	30,334	29,344	28,857	28,514	27,618	27,461	27,393	27,384	117,049	109,856	106,021
Interest expense:
Short-term debt interest expense	(137)	(114)	(110)	(107)	(77)	(72)	(57)	(44)	(468)	(250)	(206)
Long-term debt interest expense	(25,224)	(23,861)	(23,370)	(23,175)	(22,430)	(22,115)	(22,334)	(21,994)	(95,630)	(88,873)	(84,520)
Total interest expense	(25,361)	(23,975)	(23,480)	(23,282)	(22,507)	(22,187)	(22,391)	(22,038)	(96,098)	(89,123)	(84,726)
Net interest income	4,973	5,369	5,377	5,232	5,111	5,274	5,002	5,346	20,951	20,733	21,295
Benefit (provision) for credit losses	1,080	716	1,296	217	560	(182)	1,267	396	3,309	2,041	2,155
Net interest income after benefit (provision) for credit losses	6,053	6,085	6,673	5,449	5,671	5,092	6,269	5,742	24,260	22,774	23,450
Investment gains (losses), net	259	166	277	250	833	313	385	(9)	952	1,522	1,256
Fair value gains (losses), net	(539)	386	229	1,045	(191)	(289)	(691)	(40)	1,121	(1,211)	(1,081)
Fee and other income	149	271	239	320	431	1,194	353	249	979	2,227	966
Non-interest income (loss)	(131)	823	745	1,615	1,073	1,218	47	200	3,052	2,538	1,141
Administrative expenses:
Salaries and employee benefits	(350)	(355)	(365)	(381)	(321)	(331)	(332)	(344)	(1,451)	(1,328)	(1,336)
Professional services	(288)	(247)	(254)	(243)	(252)	(218)	(234)	(229)	(1,032)	(933)	(955)
Other administrative expenses	(176)	(138)	(136)	(126)	(130)	(115)	(120)	(111)	(576)	(476)	(450)
Total administrative expenses	(814)	(740)	(755)	(750)	(703)	(664)	(686)	(684)	(3,059)	(2,737)	(2,741)
Foreclosed property expense	(157)	(159)	(139)	(162)	(130)	(140)	(34)	(217)	(617)	(521)	(644)
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees	(586)	(576)	(565)	(557)	(544)	(531)	(518)	(503)	(2,284)	(2,096)	(1,845)
Other expenses, net	(307)	(377)	(366)	(203)	(411)	(427)	(291)	(382)	(1,253)	(1,511)	(1,028)
Total expenses	(1,864)	(1,852)	(1,825)	(1,672)	(1,788)	(1,762)	(1,529)	(1,786)	(7,213)	(6,865)	(6,258)
Income before federal income taxes	4,058	5,056	5,593	5,392	4,956	4,548	4,787	4,156	20,099	18,447	18,333
Provision for federal income taxes	(828)	(1,045)	(1,136)	(1,131)	(11,489)	(1,525)	(1,587)	(1,383)	(4,140)	(15,984)	(6,020)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	3,230	4,011	4,457	4,261	(6,533)	3,023	3,200	2,773	15,959	2,463	12,313
Net income (loss) attributable to Fannie Mae									15,959	2,463	12,313
Dividends distributed or available for distribution to senior preferred stockholder	(3,241)	(3,975)	(4,459)	(938)	0	(3,048)	(3,117)	(2,779)	(12,613)	(8,944)	(12,236)
Net income (loss) attributable to common stockholders	$ (11)	$ 36	$ (2)	$ 3,323	$ (6,533)	$ (25)	$ 83	$ (6)	$ 3,346	$ (6,481)	$ 77
Earnings (loss) per share: Basic	$ 0.00	$ 0.01	$ 0.00	$ 0.58	$ (1.13)	$ 0.00	$ 0.01	$ 0.00	$ 0.58	$ (1.12)	$ 0.01
Earnings (loss) per share: Diluted	$ 0.00	$ 0.01	$ 0.00	$ 0.56	$ (1.13)	$ 0.00	$ 0.01	$ 0.00	$ 0.57	$ (1.12)	$ 0.01
Weighted-average common shares outstanding: Basic	5,762	5,762	5,762	5,762	5,762	5,762	5,762	5,762	5,762	5,762	5,762
Weighted-average common shares outstanding: Diluted	5,762	5,893	5,762	5,893	5,762	5,762	5,893	5,762	5,893	5,762	5,893