Fair Value Recurring Changes in Fair Value (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Other assets [Abstract]
Derivative assets, gross amount offset	$ (5,626)	$ (5,186)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(12,036)	(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	7,920	6,403
Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	15,570	15,629
Liabilities [Abstract]
Long-term debt, fair value	21,373	19,483
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading Securities	19,367	19,466
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Total assets at fair value	65,716	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	9,633	9,008
Available-for-sale securities	14,261	15,574
Mortgage loans held for investment, at fair value	2,675,585	2,687,759
Total assets at fair value	2,751,816	2,771,628
Other assets [Abstract]
Derivatives assets at fair value	7,103	6,489
Liabilities [Abstract]
Total liabilities at fair value	3,306,127	3,299,289
Other liabilities [Abstract]
Derivative liabilities at fair value	12,689	10,671
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	31,127	29,896
Liabilities [Abstract]
Long-term debt, fair value	363,358	367,703
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	2,644,458	2,657,863
Liabilities [Abstract]
Long-term debt, fair value	2,830,632	2,815,843
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading Securities	2,286	3,030
Available-for-sale securities	13,471	15,080
Mortgage loans held for investment, at fair value	419,199	400,327
Total assets at fair value	436,780	419,874
Other assets [Abstract]
Derivatives assets at fair value	215	182
Liabilities [Abstract]
Total liabilities at fair value	26,968	23,592
Other liabilities [Abstract]
Derivative liabilities at fair value	149	137
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	214,493	217,064
Liabilities [Abstract]
Long-term debt, fair value	994	982
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	204,706	183,263
Liabilities [Abstract]
Long-term debt, fair value	22,826	19,334
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading Securities	31,286	31,504
Available-for-sale securities	27,732	30,654
Total assets at fair value	76,280	79,272
Other assets [Abstract]
Derivatives assets at fair value	1,692	1,485
Derivative assets, gross amount offset	(5,626)	(5,186)
Liabilities [Abstract]
Long-term debt, fair value	29,293	25,886
Total liabilities at fair value	30,095	26,500
Other liabilities [Abstract]
Derivative liabilities at fair value	802	614
Derivative liabilities, gross amount offset	(12,036)	(10,194)
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	6,827	6,235
Other liabilities [Abstract]
Derivative liabilities at fair value	11,223	9,472
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	52	153
Other liabilities [Abstract]
Derivative liabilities at fair value	1,047	991
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	27	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	412	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	7,920	6,403
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior Floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	7,920	6,403
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	15,570	15,629
Liabilities [Abstract]
Long-term debt, fair value	21,373	19,483
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	4,959	4,940
Available-for-sale securities	5,407	5,639
Recurring Fair Value Measurements [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	1,723	1,369
Available-for-sale securities	5,257	5,528
Recurring Fair Value Measurements [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	214	166
Available-for-sale securities	458	476
Recurring Fair Value Measurements [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	833	920
Available-for-sale securities	4,464	5,678
Recurring Fair Value Measurements [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	876	1,307
Available-for-sale securities	4,608	5,240
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading Securities	2,478	2,515
Available-for-sale securities	1,370	1,397
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading Securities	742	722
Available-for-sale securities	3,560	4,023
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Trading Securities	94	99
Available-for-sale securities	2,608	2,673
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	19,367	19,466
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading Securities	19,367	19,466
Available-for-sale securities	0	0
Total assets at fair value	19,367	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]
Assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
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	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	19,367	19,466
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading Securities	9,633	9,008
Available-for-sale securities	14,261	15,574
Total assets at fair value	44,757	44,867
Other assets [Abstract]
Derivatives assets at fair value	7,103	6,489
Liabilities [Abstract]
Long-term debt, fair value	28,355	24,996
Total liabilities at fair value	41,044	35,667
Other liabilities [Abstract]
Derivative liabilities at fair value	12,689	10,671
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	6,666	6,085
Other liabilities [Abstract]
Derivative liabilities at fair value	11,078	9,339
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	52	153
Other liabilities [Abstract]
Derivative liabilities at fair value	1,047	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	385	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	7,529	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	7,529	6,040
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	13,760	13,796
Liabilities [Abstract]
Long-term debt, fair value	20,826	18,956
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	4,957	4,635
Available-for-sale securities	5,202	5,639
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading Securities	1,723	1,369
Available-for-sale securities	5,252	5,522
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading Securities	214	166
Available-for-sale securities	458	476
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading Securities	261	323
Available-for-sale securities	1,978	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	2,478	2,515
Available-for-sale securities	1,370	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	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	2,286	3,030
Available-for-sale securities	13,471	15,080
Total assets at fair value	17,782	20,125
Other assets [Abstract]
Derivatives assets at fair value	215	182
Liabilities [Abstract]
Long-term debt, fair value	938	890
Total liabilities at fair value	1,087	1,027
Other liabilities [Abstract]
Derivative liabilities at fair value	149	137
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	161	150
Other liabilities [Abstract]
Derivative liabilities at fair value	145	133
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [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] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	27	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	27	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	391	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	391	363
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	1,810	1,833
Liabilities [Abstract]
Long-term debt, fair value	547	527
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading Securities	2	305
Available-for-sale securities	205	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	5	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	572	597
Available-for-sale securities	2,486	3,140
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading Securities	876	1,307
Available-for-sale securities	4,608	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	742	722
Available-for-sale securities	3,560	4,023
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Trading Securities	94	99
Available-for-sale securities	2,607	2,671
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading Securities	$ 0	$ 0