Selected Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest income:
Trading securities	$ 132	$ 151	$ 143	$ 127	$ 146	$ 185	$ 222	$ 226	$ 553	$ 779	$ 989
Available-for-sale securities	373	395	414	440	487	546	651	673	1,622	2,357	3,299
Mortgage loans	27,588	27,779	28,165	28,588	28,659	28,299	28,056	29,224	112,120	114,238	124,706
Other	33	29	24	24	32	37	49	57	110	175	196
Total interest income	28,126	28,354	28,746	29,179	29,324	29,067	28,978	30,180	114,405	117,549	129,190
Interest expense:
Short-term debt	27	26	21	20	22	29	37	43	94	131	152
Long-term debt	22,957	23,144	23,821	24,421	24,451	23,456	23,274	23,833	94,343	95,014	107,537
Total interest expense	22,984	23,170	23,842	24,441	24,473	23,485	23,311	23,876	94,437	95,145	107,689
Net interest income (loss)	5,142	5,184	4,904	4,738	4,851	5,582	5,667	6,304	19,968	22,404	21,501
Benefit for credit losses	466	1,085	1,639	774	0	2,609	5,383	957	3,964	8,949	852
Net interest income after benefit for credit losses	5,608	6,269	6,543	5,512	4,851	8,191	11,050	7,261	23,932	31,353	22,353
Investment gains, net	187	171	483	95	113	621	284	109	936	1,127	(226)
Fair value gains, net	(2,502)	(207)	(934)	(1,190)	961	335	829	834	(4,833)	2,959	(2,977)
Debt extinguishment (losses) gains, net	17	11	38	0	35	92	27	(23)	66	131	(244)
Fee and other income	323	826	383	4,355	2,136	741	485	568	5,887	3,930	1,487
Non-interest income (loss)	(1,975)	801	(30)	3,260	3,245	1,789	1,625	1,488	2,056	8,147	(1,960)
Administrative expenses:
Salaries and employee benefits	340	337	319	325	290	307	304	317	1,321	1,218	1,195
Professional services	296	263	275	242	232	236	219	223	1,076	910	766
Occupancy expenses	59	47	47	50	48	48	47	46	203	189	188
Other administrative expenses	7	59	56	55	62	55	56	55	177	228	218
Total administrative expenses	702	706	697	672	632	646	626	641	2,777	2,545	2,367
Foreclosed property expense (income)	369	249	(214)	(262)	(1,082)	(1,165)	(332)	(260)	142	(2,839)	(254)
Other expenses (income), net	65	72	276	131	(34)	124	68	68	544	226	822
Total expenses (income)	1,503	1,378	1,094	863	(178)	(119)	595	635	4,838	933	3,173
Income before federal income taxes	2,130	5,692	5,419	7,909	8,274	10,099	12,080	8,114	21,150	38,567	17,220
(Provision) benefit for federal income taxes	(818)	(1,787)	(1,752)	(2,584)	(1,816)	(1,355)	(1,985)	50,571	(6,941)	45,415	0
Net income	1,312	3,905	3,667	5,325	6,458	8,744	10,095	58,685	14,209	83,982	17,220
Less: Net (income) loss attributable to noncontrolling interest	0	0	(1)	0	(1)	(7)	(11)	0	(1)	(19)	4
Net income attributable to Fannie Mae	1,312	3,905	3,666	5,325	6,457	8,737	10,084	58,685	14,208	83,963	17,224
Dividends distributed or available for distribution to senior preferred stockholder (Note 11)	(1,920)	(3,999)	(3,712)	(5,692)	(7,191)	(8,617)	(10,243)	(59,368)	(15,323)	(85,419)	(15,827)
Net (loss) income attributable to common stockholders (Note 11)	(608)	(94)	(46)	(367)	(734)	120	(159)	(683)	(1,115)	(1,456)	1,397
(Loss) per share:
Basic and Diluted (per share)	$ (0.11)	$ (0.02)	$ (0.01)	$ (0.06)
Basic (per share)					$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ (0.19)	$ (0.25)	$ 0.24
Diluted (per share)					$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ (0.19)	$ (0.25)	$ 0.24
Weighted-average common shares outstanding:
Basic and Diluted (in shares)	5,762	5,762	5,762	5,762
Basic (in shares)					5,762	5,762	5,762	5,762	5,762	5,762	5,762
Diluted (in shares)					5,762	5,893	5,762	5,762	5,762	5,762	5,893
US Treasury [Member]
TCCA fees	$ 367	$ 351	$ 335	$ 322	$ 306	$ 276	$ 233	$ 186	$ 1,375	$ 1,001	$ 238