Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 12,057	$ 14,075
Other assets [Abstract]
Derivative asset, gross amount offset	(4,514)	(4,024)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(6,844)	(8,650)
Mortgage commitment derivatives [Member]
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	9,582	11,133
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	36,524	23,609
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	32,317	29,485
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	73,794	64,438
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]
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	7,118	9,025
Available-for-sale securities	6,210	11,799
Mortgage loans held for investment, at fair value	2,767,813	2,702,526
Other assets [Abstract]
Derivatives assets at fair value	5,019	4,729
Total assets at fair value	2,844,940	2,770,088
Other liabilities [Abstract]
Derivatives liabilities at fair value	7,921	9,042
Total liabilities at fair value	3,243,215	3,224,091
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	36,194	26,544
Liabilities [Abstract]
Long-term debt, fair value	298,980	324,248
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	2,731,619	2,675,982
Liabilities [Abstract]
Long-term debt, fair value	2,901,316	2,819,733
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	1,127	1,398
Available-for-sale securities	2,153	8,431
Mortgage loans held for investment, at fair value	316,742	351,355
Other assets [Abstract]
Derivatives assets at fair value	182	189
Total assets at fair value	323,739	367,852
Other liabilities [Abstract]
Derivatives liabilities at fair value	138	32
Total liabilities at fair value	38,870	30,061
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	157,469	193,670
Liabilities [Abstract]
Long-term debt, fair value	770	898
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	159,273	157,685
Liabilities [Abstract]
Long-term debt, fair value	36,668	27,175
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	40,562	39,908
Available-for-sale securities	8,363	20,230
Mortgage loans held for investment, at fair value	12,057	14,075
Other assets [Abstract]
Derivatives assets at fair value	687	894
Derivative asset, gross amount offset	(4,514)	(4,024)
Total assets at fair value	61,669	75,107
Liabilities [Abstract]
Long-term debt, fair value	46,106	34,742
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,215	424
Derivative liabilities, gross amount offset	(6,844)	(8,650)
Total liabilities at fair value	47,321	35,166
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	4,296	4,697
Other liabilities [Abstract]
Derivatives liabilities at fair value	6,981	8,717
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	241	53
Other liabilities [Abstract]
Derivatives liabilities at fair value	339	197
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	33	28
Other liabilities [Abstract]
Derivatives liabilities at fair value	2	2
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	631	140
Other liabilities [Abstract]
Derivatives liabilities at fair value	737	158
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,582	11,133
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,582	11,133
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	36,524	23,609
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	4,769	4,813
Available-for-sale securities	2,554	4,221
Recurring Fair Value Measurements [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading securities	936	1,314
Available-for-sale securities	509	4,299
Recurring Fair Value Measurements [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading securities	1,122	426
Available-for-sale securities	38	391
Recurring Fair Value Measurements [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading securities	317	436
Available-for-sale securities	1,291	2,678
Recurring Fair Value Measurements [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	319	644
Available-for-sale securities	1,418	3,281
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	761	2,341
Available-for-sale securities	819	1,255
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	21	449
Available-for-sale securities	1,272	2,701
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	462	1,404
Recurring Fair Value Measurements [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	32,317	29,485
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	32,317	29,485
Available-for-sale securities	0	0
Other assets [Abstract]
Derivatives assets at fair value	0	0
Total assets at fair value	32,317	29,485
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 [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] | Mortgage commitment derivatives [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] | 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]
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	32,317	29,485
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	7,118	9,025
Available-for-sale securities	6,210	11,799
Other assets [Abstract]
Derivatives assets at fair value	5,019	4,729
Total assets at fair value	29,207	38,151
Liabilities [Abstract]
Long-term debt, fair value	45,518	33,877
Other liabilities [Abstract]
Derivatives liabilities at fair value	7,921	9,042
Total liabilities at fair value	53,439	42,919
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	4,159	4,541
Other liabilities [Abstract]
Derivatives liabilities at fair value	6,933	8,697
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	241	53
Other liabilities [Abstract]
Derivatives liabilities at fair value	339	197
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]
Derivatives 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	619	135
Other liabilities [Abstract]
Derivatives liabilities at fair value	649	148
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,235	10,764
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	9,235	10,764
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	10,860	12,598
Liabilities [Abstract]
Long-term debt, fair value	36,283	23,113
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,934	4,813
Available-for-sale securities	2,324	4,221
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading securities	936	1,314
Available-for-sale securities	504	4,295
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading securities	1,122	426
Available-for-sale securities	38	391
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading securities	46	131
Available-for-sale securities	1,119	1,637
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	319	0
Available-for-sale securities	1,373	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	761	2,341
Available-for-sale securities	819	1,255
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	33	0
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	1,127	1,398
Available-for-sale securities	2,153	8,431
Mortgage loans held for investment, at fair value	1,197	1,477
Other assets [Abstract]
Derivatives assets at fair value	182	189
Total assets at fair value	4,659	11,495
Liabilities [Abstract]
Long-term debt, fair value	588	865
Other liabilities [Abstract]
Derivatives liabilities at fair value	138	32
Total liabilities at fair value	726	897
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	137	156
Other liabilities [Abstract]
Derivatives liabilities at fair value	48	20
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 [Member]
Other assets [Abstract]
Derivatives assets at fair value	33	28
Other liabilities [Abstract]
Derivatives liabilities at fair value	2	2
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	12	5
Other liabilities [Abstract]
Derivatives liabilities at fair value	88	10
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	347	369
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	347	369
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	1,197	1,477
Liabilities [Abstract]
Long-term debt, fair value	241	496
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	835	0
Available-for-sale securities	230	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	4
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	271	305
Available-for-sale securities	172	1,041
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	0	644
Available-for-sale securities	45	3,281
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	21	449
Available-for-sale securities	1,272	2,701
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	429	1,404
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	$ 0	$ 0