Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 7,701	$ 7,825
Other assets [Abstract]
Derivative assets, gross amount offset	(2,137)	(1,288)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(2,607)	(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	4,752	5,687
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	22,855	21,880
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	44,727	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	161,035	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	8,052	8,576
Available-for-sale securities	1,521	1,612
Mortgage loans held for investment, at fair value	3,395,898	3,270,535
Other assets [Abstract]
Derivatives assets at fair value	4,661	1,360
Total assets at fair value	3,439,362	3,313,691
Other liabilities [Abstract]
Derivative liabilities at fair value	6,386	2,092
Total liabilities at fair value	3,682,046	3,506,144
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	176,634	164,144
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,440,638	3,312,763
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	162	46
Available-for-sale securities	768	792
Mortgage loans held for investment, at fair value	118,612	127,650
Other assets [Abstract]
Derivatives assets at fair value	247	199
Total assets at fair value	128,464	136,048
Other liabilities [Abstract]
Derivative liabilities at fair value	30	37
Total liabilities at fair value	34,111	32,362
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	2,215	401
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	31,767	31,827
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	52,941	48,123
Available-for-sale securities	2,289	2,404
Mortgage loans held for investment, at fair value	7,701	7,825
Other assets [Abstract]
Derivatives assets at fair value	2,771	271
Derivative assets, gross amount offset	(2,137)	(1,288)
Total assets at fair value	65,702	58,623
Liabilities [Abstract]
Long-term debt, fair value	27,607	27,567
Other liabilities [Abstract]
Derivative liabilities at fair value	3,809	435
Derivative liabilities, gross amount offset	(2,607)	(1,694)
Total liabilities at fair value	31,416	28,002
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	1,580	1,230
Other liabilities [Abstract]
Derivative liabilities at fair value	2,335	1,347
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	626	124
Other liabilities [Abstract]
Derivative liabilities at fair value	458	451
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	2,665	165
Other liabilities [Abstract]
Derivative liabilities at fair value	3,594	306
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	37	40
Other liabilities [Abstract]
Derivative liabilities at fair value	29	25
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	4,752	5,687
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	4,752	5,687
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	22,855	21,880
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,530	3,424
Available-for-sale securities	1,505	1,520
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	4,167	4,490
Available-for-sale securities	190	198
Recurring Fair Value Measurements [Member] | Private-label and other mortgage securities [Member]
Assets [Abstract]
Trading securities	444	629
Available-for-sale securities	7	57
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	296	315
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	291	314
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	44,727	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	44,727	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	44,727	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
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	44,727	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	8,052	8,576
Available-for-sale securities	1,521	1,612
Mortgage loans held for investment, at fair value	7,063	7,137
Other assets [Abstract]
Derivatives assets at fair value	4,661	1,360
Total assets at fair value	21,297	18,685
Liabilities [Abstract]
Long-term debt, fair value	27,092	27,094
Other liabilities [Abstract]
Derivative liabilities at fair value	6,386	2,092
Total liabilities at fair value	33,478	29,186
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	1,370	1,071
Other liabilities [Abstract]
Derivative liabilities at fair value	2,334	1,346
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	626	124
Other liabilities [Abstract]
Derivative liabilities at fair value	458	440
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivatives assets at fair value	2,665	165
Other liabilities [Abstract]
Derivative liabilities at fair value	3,594	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	4,320	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	4,320	5,289
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	22,772	21,805
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,462	3,379
Available-for-sale securities	1,317	1,349
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	4,167	4,489
Available-for-sale securities	190	198
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Private-label and other mortgage securities [Member]
Assets [Abstract]
Trading securities	350	629
Available-for-sale securities	7	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	162	171
Available-for-sale securities	768	792
Mortgage loans held for investment, at fair value	638	688
Other assets [Abstract]
Derivatives assets at fair value	247	199
Total assets at fair value	1,815	1,725
Liabilities [Abstract]
Long-term debt, fair value	515	473
Other liabilities [Abstract]
Derivative liabilities at fair value	30	37
Total liabilities at fair value	545	510
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	210	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	37	40
Other liabilities [Abstract]
Derivative liabilities at fair value	29	25
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	432	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	432	398
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	83	75
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	68	45
Available-for-sale securities	188	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	94	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	296	315
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	284	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