Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest income:
Trading securities	$ 128	$ 140	$ 128	$ 120	$ 114	$ 99	$ 116	$ 115	$ 516	$ 444	$ 553
Available-for-sale securities	113	134	170	203	225	261	294	376	620	1,156	1,622
Mortgage loans interest income	25,814	25,611	26,256	26,961	26,993	26,980	26,682	27,044	104,642	107,699	112,120
Other	83	66	46	48	39	37	34	33	243	143	110
Total interest income	26,138	25,951	26,600	27,332	27,371	27,377	27,126	27,568	106,021	109,442	114,405
Interest expense:
Short-term debt interest expense	42	56	57	51	47	37	33	29	206	146	94
Long-term debt interest expense	20,291	20,460	21,257	22,512	22,247	21,752	21,416	22,472	84,520	87,887	94,343
Total interest expense	20,333	20,516	21,314	22,563	22,294	21,789	21,449	22,501	84,726	88,033	94,437
Net interest income	5,805	5,435	5,286	4,769	5,077	5,588	5,677	5,067	21,295	21,409	19,968
Benefit (provision) for credit losses	(1,303)	673	1,601	1,184	(255)	1,550	(1,033)	533	2,155	795	3,964
Net interest income after benefit (provision) for credit losses	4,502	6,108	6,887	5,953	4,822	7,138	4,644	5,600	23,450	22,204	23,932
Investment gains, net	322	467	398	69	181	299	514	342	1,256	1,336	936
Fair value gains (losses), net	3,890	(491)	(1,667)	(2,813)	135	(2,589)	2,606	(1,919)	(1,081)	(1,767)	(4,833)
Fee and other income	414	175	174	203	225	259	556	308	966	1,348	5,887
Non-interest income (loss)	4,626	151	(1,095)	(2,541)	541	(2,031)	3,676	(1,269)	1,141	917	1,990
Administrative expenses:
Salaries and employee benefits	319	322	331	364	320	317	331	351	1,336	1,319	1,321
Professional services	271	237	232	215	243	219	251	271	955	984	1,076
Occupancy expenses	50	45	46	45	53	43	43	43	186	182	203
Other administrative expenses	74	57	69	64	70	373	64	58	264	565	177
Total administrative expenses	714	661	678	688	686	952	689	723	2,741	3,050	2,777
Foreclosed property expense	137	110	63	334	477	497	182	473	644	1,629	142
TCCA fees	487	465	453	440	429	413	397	382	1,845	1,621	1,375
Other expenses, net	210	300	254	264	201	215	202	(5)	1,028	613	478
Total expenses	1,548	1,536	1,448	1,726	1,793	2,077	1,470	1,573	6,258	6,913	4,772
Income before federal income taxes	7,580	4,723	4,344	1,686	3,570	3,030	6,850	2,758	18,333	16,208	21,150
Provision for federal income taxes	(2,545)	(1,527)	(1,398)	(550)	(1,103)	(1,070)	(2,210)	(870)	(6,020)	(5,253)	(6,941)
Net income					2,467	1,960	4,640	1,888	12,313	10,955	14,209
Less: Net income attributable to noncontrolling interest					(1)	0	0	0	0	(1)	(1)
Net income attributable to Fannie Mae	5,035	3,196	2,946	1,136	2,466	1,960	4,640	1,888	12,313	10,954	14,208
Dividends distributed or available for distribution to senior preferred stockholder	(5,471)	(2,977)	(2,869)	(919)	(2,859)	(2,202)	(4,359)	(1,796)	(12,236)	(11,216)	(15,323)
Net income (loss) attributable to common stockholders (Note 11)	$ (436)	$ 219	$ 77	$ 217	$ (393)	$ (242)	$ 281	$ 92	$ 77	$ (262)	$ (1,115)
Earnings (loss) per share: Basic	$ (0.08)	$ 0.04	$ 0.01	$ 0.04	$ (0.07)	$ (0.04)	$ 0.05	$ 0.02	$ 0.01	$ (0.05)	$ (0.19)
Earnings (loss) per share: Diluted	$ (0.08)	$ 0.04	$ 0.01	$ 0.04	$ (0.07)	$ (0.04)	$ 0.05	$ 0.02	$ 0.01	$ (0.05)	$ (0.19)
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	5,893	5,893	5,762	5,762	5,893	5,893	5,893	5,762	5,762