Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 7,303	$ 7,825
Other assets [Abstract]
Derivative assets, gross amount offset	(2,108)	(1,288)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(2,396)	(1,694)
Mortgage commitment derivatives
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	4,546	5,687
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,883	21,880
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	96,037	39,501
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	202,986	89,558
Other liabilities [Abstract]
Derivative 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	10,571	8,576
Available-for-sale securities	1,140	1,612
Mortgage loans held for investment, at fair value	3,400,773	3,270,535
Other assets [Abstract]
Derivatives assets at fair value	2,698	1,360
Total assets at fair value	3,461,999	3,313,691
Other liabilities [Abstract]
Derivative liabilities at fair value	3,731	2,092
Total liabilities at fair value	3,856,250	3,506,144
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	244,967	164,144
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,565,283	3,312,763
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	136	46
Available-for-sale securities	980	792
Mortgage loans held for investment, at fair value	195,726	127,650
Other assets [Abstract]
Derivatives assets at fair value	314	199
Total assets at fair value	205,309	136,048
Other liabilities [Abstract]
Derivative liabilities at fair value	45	37
Total liabilities at fair value	32,723	32,362
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	860	401
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	31,733	31,827
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	106,744	48,123
Available-for-sale securities	2,120	2,404
Mortgage loans held for investment, at fair value	7,303	7,825
Other assets [Abstract]
Derivatives assets at fair value	904	271
Derivative assets, gross amount offset	(2,108)	(1,288)
Total assets at fair value	124,738	58,623
Liabilities [Abstract]
Long-term debt, fair value	28,429	27,567
Other liabilities [Abstract]
Derivative liabilities at fair value	1,380	435
Derivative liabilities, gross amount offset	(2,396)	(1,694)
Total liabilities at fair value	29,809	28,002
Cash and Cash Equivalents, Fair Value Disclosure	7,667
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	1,506	1,230
Other liabilities [Abstract]
Derivative liabilities at fair value	2,034	1,347
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	654	124
Other liabilities [Abstract]
Derivative liabilities at fair value	474	451
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	767	165
Other liabilities [Abstract]
Derivative liabilities at fair value	1,224	306
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	85	40
Other liabilities [Abstract]
Derivative liabilities at fair value	44	25
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	4,546	5,687
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	4,546	5,687
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,883	21,880
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	7,053	3,424
Available-for-sale securities	1,394	1,520
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,122	4,490
Available-for-sale securities	180	198
Recurring Fair Value Measurements [Member] | Private-label and other mortgage securities [Member]
Assets [Abstract]
Trading securities	459	629
Available-for-sale securities	7	57
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	278	315
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	261	314
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	96,037	39,501
Recurring Fair Value Measurements [Member] | Other securities [Member]
Assets [Abstract]
Trading securities	73	79
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	96,037	39,501
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	103,704	39,501
Liabilities [Abstract]
Long-term debt, fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Total liabilities at fair value	0	0
Cash and Cash Equivalents, Fair Value Disclosure	7,667
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] | Mortgage commitment derivatives
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] | 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] | Private-label and other mortgage 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] | Mortgage revenue bonds [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] | 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	96,037	39,501
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other securities [Member]
Assets [Abstract]
Trading securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	10,571	8,576
Available-for-sale securities	1,140	1,612
Mortgage loans held for investment, at fair value	6,506	7,137
Other assets [Abstract]
Derivatives assets at fair value	2,698	1,360
Total assets at fair value	20,915	18,685
Liabilities [Abstract]
Long-term debt, fair value	27,907	27,094
Other liabilities [Abstract]
Derivative liabilities at fair value	3,731	2,092
Total liabilities at fair value	31,638	29,186
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	1,277	1,071
Other liabilities [Abstract]
Derivative liabilities at fair value	2,033	1,346
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	654	124
Other liabilities [Abstract]
Derivative liabilities at fair value	474	440
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	767	165
Other liabilities [Abstract]
Derivative liabilities at fair value	1,224	306
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] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	4,113	5,289
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	4,113	5,289
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,794	21,805
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	7,015	3,379
Available-for-sale securities	948	1,349
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,122	4,489
Available-for-sale securities	180	198
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Private-label and other mortgage securities [Member]
Assets [Abstract]
Trading securities	361	629
Available-for-sale securities	5	57
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
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	7	8
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 Other Observable Inputs (Level 2) [Member] | Other securities [Member]
Assets [Abstract]
Trading securities	73	79
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	136	171
Available-for-sale securities	980	792
Mortgage loans held for investment, at fair value	797	688
Other assets [Abstract]
Derivatives assets at fair value	314	199
Total assets at fair value	2,227	1,725
Liabilities [Abstract]
Long-term debt, fair value	522	473
Other liabilities [Abstract]
Derivative liabilities at fair value	45	37
Total liabilities at fair value	567	510
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	229	159
Other liabilities [Abstract]
Derivative liabilities at fair value	1	1
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	11
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives
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	85	40
Other liabilities [Abstract]
Derivative liabilities at fair value	44	25
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	433	398
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	433	398
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	89	75
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	38	45
Available-for-sale securities	446	171
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	0	1
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Private-label and other mortgage securities [Member]
Assets [Abstract]
Trading securities	98	0
Available-for-sale securities	2	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	278	315
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	254	306
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other securities [Member]
Assets [Abstract]
Trading securities	$ 0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Trading Securities
Assets [Abstract]
Trading securities		$ 46