Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 14,605	$ 15,629
Other assets [Abstract]
Derivative assets, gross amount offset	(4,417)	(5,186)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(10,635)	(10,194)
Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivative assets, gross amount offset	0	0
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	0	0
Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,975	6,403
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,143	19,483
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading Securities	26,961	19,466
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Total assets at fair value	61,657	57,431
Other assets [Abstract]
Derivatives assets at fair value	0	0
Liabilities [Abstract]
Total liabilities at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading Securities	9,458	9,008
Available-for-sale securities	12,455	15,574
Mortgage loans held for investment, at fair value	2,701,802	2,687,759
Total assets at fair value	2,776,652	2,771,628
Other assets [Abstract]
Derivatives assets at fair value	5,830	6,489
Liabilities [Abstract]
Total liabilities at fair value	3,268,718	3,299,289
Other liabilities [Abstract]
Derivative liabilities at fair value	11,402	10,671
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	26,358	29,896
Liabilities [Abstract]
Long-term debt, fair value	334,007	367,703
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	2,675,444	2,657,863
Liabilities [Abstract]
Long-term debt, fair value	2,827,733	2,815,843
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading Securities	1,590	3,030
Available-for-sale securities	9,552	15,080
Mortgage loans held for investment, at fair value	386,291	400,327
Total assets at fair value	402,365	419,874
Other assets [Abstract]
Derivatives assets at fair value	197	182
Liabilities [Abstract]
Total liabilities at fair value	30,414	23,592
Other liabilities [Abstract]
Derivative liabilities at fair value	137	137
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	201,369	217,064
Liabilities [Abstract]
Long-term debt, fair value	931	982
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	184,922	183,263
Liabilities [Abstract]
Long-term debt, fair value	26,613	19,334
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading Securities	38,009	31,504
Available-for-sale securities	22,007	30,654
Mortgage loans held for investment, at fair value	14,605	15,629
Total assets at fair value	76,231	79,272
Other assets [Abstract]
Derivatives assets at fair value	1,610	1,485
Derivative assets, gross amount offset	(4,417)	(5,186)
Liabilities [Abstract]
Long-term debt, fair value	33,118	25,886
Total liabilities at fair value	34,022	26,500
Other liabilities [Abstract]
Derivative liabilities at fair value	904	614
Derivative liabilities, gross amount offset	(10,635)	(10,194)
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	5,575	6,235
Other liabilities [Abstract]
Derivative liabilities at fair value	10,664	9,472
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	43	153
Other liabilities [Abstract]
Derivative liabilities at fair value	307	991
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	25	28
Other liabilities [Abstract]
Derivative liabilities at fair value	1	1
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	384	255
Other liabilities [Abstract]
Derivative liabilities at fair value	567	344
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,975	6,403
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior Floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,975	6,403
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,143	19,483
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	5,097	4,940
Available-for-sale securities	4,282	5,639
Recurring Fair Value Measurements [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	1,474	1,369
Available-for-sale securities	4,599	5,528
Recurring Fair Value Measurements [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	353	166
Available-for-sale securities	413	476
Recurring Fair Value Measurements [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	451	920
Available-for-sale securities	3,349	5,678
Recurring Fair Value Measurements [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	696	1,307
Available-for-sale securities	3,677	5,240
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading Securities	2,398	2,515
Available-for-sale securities	1,309	1,397
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading Securities	579	722
Available-for-sale securities	2,933	4,023
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Trading Securities		99
Available-for-sale securities	1,445	2,673
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	26,961	19,466
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading Securities	26,961	19,466
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Total assets at fair value	26,961	19,466
Other assets [Abstract]
Derivatives assets at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Total liabilities at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member] | Senior Floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | CMBS [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Assets [Abstract]
Trading Securities		0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	26,961	19,466
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading Securities	9,458	9,008
Available-for-sale securities	12,455	15,574
Mortgage loans held for investment, at fair value	13,105	13,796
Total assets at fair value	40,848	44,867
Other assets [Abstract]
Derivatives assets at fair value	5,830	6,489
Liabilities [Abstract]
Long-term debt, fair value	32,178	24,996
Total liabilities at fair value	43,580	35,667
Other liabilities [Abstract]
Derivative liabilities at fair value	11,402	10,671
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	5,417	6,085
Other liabilities [Abstract]
Derivative liabilities at fair value	10,531	9,339
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	39	153
Other liabilities [Abstract]
Derivative liabilities at fair value	307	991
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	374	251
Other liabilities [Abstract]
Derivative liabilities at fair value	564	341
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,606	6,040
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member] | Senior Floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,606	6,040
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	22,572	18,956
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	5,097	4,635
Available-for-sale securities	4,282	5,639
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	1,474	1,369
Available-for-sale securities	4,595	5,522
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	353	166
Available-for-sale securities	413	476
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	136	323
Available-for-sale securities	1,855	2,538
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Trading Securities	2,398	2,515
Available-for-sale securities	1,309	1,397
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other [Member]
Assets [Abstract]
Trading Securities		0
Available-for-sale securities	1	2
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading Securities	1,590	3,030
Available-for-sale securities	9,552	15,080
Mortgage loans held for investment, at fair value	1,500	1,833
Total assets at fair value	12,839	20,125
Other assets [Abstract]
Derivatives assets at fair value	197	182
Liabilities [Abstract]
Long-term debt, fair value	940	890
Total liabilities at fair value	1,077	1,027
Other liabilities [Abstract]
Derivative liabilities at fair value	137	137
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	158	150
Other liabilities [Abstract]
Derivative liabilities at fair value	133	133
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	4	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	25	28
Other liabilities [Abstract]
Derivative liabilities at fair value	1	1
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	10	4
Other liabilities [Abstract]
Derivative liabilities at fair value	3	3
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	369	363
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior Floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	369	363
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	571	527
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	0	305
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	4	6
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	315	597
Available-for-sale securities	1,494	3,140
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	696	1,307
Available-for-sale securities	3,677	5,240
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | CMBS [Member]
Assets [Abstract]
Trading Securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading Securities	579	722
Available-for-sale securities	2,933	4,023
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Trading Securities		99
Available-for-sale securities	1,444	2,671
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	$ 0	$ 0