Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest income:
Trading securities	$ 114	$ 99	$ 116	$ 115	$ 132	$ 151	$ 143	$ 127	$ 444	$ 553	$ 779
Available-for-sale securities	225	261	294	376	373	395	414	440	1,156	1,622	2,357
Mortgage loans interest income	26,993	26,980	26,682	27,044	27,588	27,779	28,165	28,588	107,699	112,120	114,238
Other	39	37	34	33	33	29	24	24	143	110	175
Total interest income	27,371	27,377	27,126	27,568	28,126	28,354	28,746	29,179	109,442	114,405	117,549
Interest expense:
Short-term debt interest expense	47	37	33	29	27	26	21	20	146	94	131
Long-term debt interest expense	22,247	21,752	21,416	22,472	22,957	23,144	23,821	24,421	87,887	94,343	95,014
Total interest expense	22,294	21,789	21,449	22,501	22,984	23,170	23,842	24,441	88,033	94,437	95,145
Net interest income (loss)	5,077	5,588	5,677	5,067	5,142	5,184	4,904	4,738	21,409	19,968	22,404
Benefit (provision) for credit losses	(255)	1,550	(1,033)	533	466	1,085	1,639	774	795	3,964	8,949
Net interest income after benefit (provision) for credit losses	4,822	7,138	4,644	5,600	5,608	6,269	6,543	5,512	22,204	23,932	31,353
Investment gains, net	181	299	514	342	187	171	483	95	1,336	936	1,127
Fair value gains (losses), net	135	(2,589)	2,606	(1,919)	(2,502)	(207)	(934)	(1,190)	(1,767)	(4,833)	2,959
Fee and other income	225	259	556	308	323	826	383	4,355	1,348	5,887	3,930
Non-interest income (loss)	541	(2,031)	3,676	(1,269)	(1,992)	790	(68)	3,260	917	1,990	8,016
Administrative expenses:
Salaries and employee benefits	320	317	331	351	340	337	319	325	1,319	1,321	1,218
Professional services	243	219	251	271	296	263	275	242	984	1,076	910
Occupancy expenses	53	43	43	43	59	47	47	50	182	203	189
Other administrative expenses	70	373	64	58	7	59	56	55	565	177	228
Total administrative expenses	686	952	689	723	702	706	697	672	3,050	2,777	2,545
Foreclosed property expense (income)	477	497	182	473	369	249	(214)	(262)	1,629	142	(2,839)
TCCA fees	429	413	397	382	367	351	335	322	1,621	1,375	1,001
Other expenses (income), net	201	215	202	(5)	48	61	238	131
Total expenses	1,793	2,077	1,470	1,573	1,486	1,367	1,056	863	6,913	4,772	802
Income before federal income taxes	3,570	3,030	6,850	2,758	2,130	5,692	5,419	7,909	16,208	21,150	38,567
Provision for federal income taxes	(1,103)	(1,070)	(2,210)	(870)	(818)	(1,787)	(1,752)	(2,584)	(5,253)	(6,941)	45,415
Net income	2,467	1,960	4,640	1,888	1,312	3,905	3,667	5,325	10,955	14,209	83,982
Less: Net income attributable to noncontrolling interest	(1)	0	0	0	0	0	(1)	0	(1)	(1)	(19)
Net income attributable to Fannie Mae	2,466	1,960	4,640	1,888	1,312	3,905	3,666	5,325	10,954	14,208	83,963
Dividends distributed or available for distribution to senior preferred stockholder	(2,859)	(2,202)	(4,359)	(1,796)	(1,920)	(3,999)	(3,712)	(5,692)	(11,216)	(15,323)	(85,419)
Net income (loss) attributable to common stockholders (Note 11)	$ (393)	$ (242)	$ 281	$ 92	$ (608)	$ (94)	$ (46)	$ (367)	$ (262)	$ (1,115)	$ (1,456)
Earnings (loss) per share: Basic and Diluted (in dollars per share)	$ (0.07)	$ (0.04)	$ 0.05	$ 0.02	$ (0.11)	$ (0.02)	$ (0.01)	$ (0.06)	$ (0.05)	$ (0.19)	$ (0.25)
Weighted average common shares outstanding: Basic	5,762	5,762	5,762	5,762
Weighted-average common shares outstanding: Diluted	5,762	5,762	5,893	5,893
Weighted-average common shares outstanding: Basic and Diluted					5,762	5,762	5,762	5,762	5,762	5,762	5,762