Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 10,596	$ 12,057
Other assets [Abstract]
Derivative asset, gross amount offset	(4,272)	(4,514)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(4,979)	(6,844)
Mortgage commitment derivatives
Other assets [Abstract]
Derivative asset, 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	8,186	9,582
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	30,493	36,524
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	29,222	32,317
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivatives assets at fair value	0	0
Total assets at fair value	64,282	73,794
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Total liabilities at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	4,256	7,118
Available-for-sale securities	3,530	6,210
Mortgage loans held for investment, at fair value	2,886,470	2,767,813
Other assets [Abstract]
Derivatives assets at fair value	4,274	5,019
Total assets at fair value	2,948,237	2,844,940
Other liabilities [Abstract]
Derivatives liabilities at fair value	5,272	7,921
Total liabilities at fair value	3,302,681	3,243,215
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	249,780	298,980
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,014,250	2,901,316
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	1,201	1,127
Available-for-sale securities	1,313	2,153
Mortgage loans held for investment, at fair value	315,719	316,742
Other assets [Abstract]
Derivatives assets at fair value	169	182
Total assets at fair value	324,173	323,739
Other liabilities [Abstract]
Derivatives liabilities at fair value	35	138
Total liabilities at fair value	42,389	38,870
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	837	770
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	40,683	36,668
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	34,679	40,562
Available-for-sale securities	4,843	8,363
Mortgage loans held for investment, at fair value	10,596	12,057
Other assets [Abstract]
Derivatives assets at fair value	171	687
Derivative asset, gross amount offset	(4,272)	(4,514)
Total assets at fair value	50,289	61,669
Liabilities [Abstract]
Long-term debt, fair value	38,679	46,106
Other liabilities [Abstract]
Derivatives liabilities at fair value	328	1,215
Derivative liabilities, gross amount offset	(4,979)	(6,844)
Total liabilities at fair value	39,007	47,321
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	4,181	4,296
Other liabilities [Abstract]
Derivatives liabilities at fair value	4,754	6,981
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	108	241
Other liabilities [Abstract]
Derivatives liabilities at fair value	324	339
Recurring Fair Value Measurements [Member] | Other
Other assets [Abstract]
Derivatives assets at fair value	22	33
Other liabilities [Abstract]
Derivatives liabilities at fair value	1	2
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	132	631
Other liabilities [Abstract]
Derivatives liabilities at fair value	228	737
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	8,186	9,582
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	8,186	9,582
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	30,493	36,524
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,876	4,769
Available-for-sale securities	2,119	2,554
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	1,118	2,058
Available-for-sale securities	357	547
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	453	636
Available-for-sale securities	1,314	2,709
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	9	761
Available-for-sale securities	15	819
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	1	21
Available-for-sale securities	671	1,272
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	367	462
Recurring Fair Value Measurements [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	29,222	32,317
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	29,222	32,317
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivatives assets at fair value	0	0
Total assets at fair value	29,222	32,317
Liabilities [Abstract]
Long-term debt, fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Total 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]
Derivatives 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]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivatives 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
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivatives 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] | Other agency [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 and 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]
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	29,222	32,317
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	4,256	7,118
Available-for-sale securities	3,530	6,210
Mortgage loans held for investment, at fair value	9,480	10,860
Other assets [Abstract]
Derivatives assets at fair value	4,274	5,019
Total assets at fair value	21,540	29,207
Liabilities [Abstract]
Long-term debt, fair value	37,721	45,518
Other liabilities [Abstract]
Derivatives liabilities at fair value	5,272	7,921
Total liabilities at fair value	42,993	53,439
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	4,035	4,159
Other liabilities [Abstract]
Derivatives liabilities at fair value	4,721	6,933
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	108	241
Other liabilities [Abstract]
Derivatives liabilities at fair value	324	339
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	131	619
Other liabilities [Abstract]
Derivatives liabilities at fair value	227	649
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	7,810	9,235
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	7,810	9,235
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	29,911	36,283
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	2,905	3,934
Available-for-sale securities	1,911	2,324
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	1,083	2,058
Available-for-sale securities	357	542
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	259	365
Available-for-sale securities	1,237	2,492
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	9	761
Available-for-sale securities	15	819
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]
Available-for-sale securities	10	33
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	1,201	1,127
Available-for-sale securities	1,313	2,153
Mortgage loans held for investment, at fair value	1,116	1,197
Other assets [Abstract]
Derivatives assets at fair value	169	182
Total assets at fair value	3,799	4,659
Liabilities [Abstract]
Long-term debt, fair value	958	588
Other liabilities [Abstract]
Derivatives liabilities at fair value	35	138
Total liabilities at fair value	993	726
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	146	137
Other liabilities [Abstract]
Derivatives liabilities at fair value	33	48
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]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other
Other assets [Abstract]
Derivatives assets at fair value	22	33
Other liabilities [Abstract]
Derivatives liabilities at fair value	1	2
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	1	12
Other liabilities [Abstract]
Derivatives liabilities at fair value	1	88
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	376	347
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	376	347
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	582	241
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	971	835
Available-for-sale securities	208	230
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	35	0
Available-for-sale securities	0	5
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	194	271
Available-for-sale securities	77	217
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	1	21
Available-for-sale securities	671	1,272
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	357	429
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	$ 0	$ 0