Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets:
Total Fair Value	$ 535	$ 564
Mortgage loans held for investment, net of allowance for loan losses	3,176	3,315
Derivative assets at fair value	102	202
Derivative assets, gross amount offset	(333)	(283)
Liabilities:
Derivative liabilities	78	140
Fannie Mae
Liabilities:
Long-term debt:	654	761
Consolidated Trusts
Liabilities:
Long-term debt:	13,762	14,343
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	47,626	47,764
Total Fair Value	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Derivative assets at fair value	0	0
Total financial assets	80,880	81,745
Liabilities:
Derivative liabilities	0	0
Total financial liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities:
Long-term debt:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities:
Long-term debt:	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	1,707	4,762
Total Fair Value	48	50
Mortgage loans held for investment, net of allowance for loan losses	3,494,262	3,571,555
Derivative assets at fair value	353	395
Total financial assets	3,572,424	3,643,669
Liabilities:
Derivative liabilities	3,331	3,327
Total financial liabilities	3,690,331	3,760,502
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	104,063	106,701
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,568,654	3,633,157
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	26
Total Fair Value	487	514
Mortgage loans held for investment, net of allowance for loan losses	130,364	130,022
Derivative assets at fair value	82	90
Total financial assets	132,907	133,003
Liabilities:
Derivative liabilities	13	13
Total financial liabilities	959	976
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	600	605
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	285	293
Recurring
Assets:
Trading securities	49,361	52,552
Total Fair Value	535	564
Mortgage loans held for investment, net of allowance for loan losses	3,176	3,315
Derivative assets at fair value	102	202
Derivative assets, gross amount offset	(333)	(283)
Total financial assets	53,174	56,633
Liabilities:
Long-term debt:	14,416	15,104
Derivative liabilities	78	140
Derivative liabilities, gross amount offset	(3,266)	(3,200)
Total financial liabilities	14,494	15,244
Recurring | Fannie Mae
Liabilities:
Long-term debt:	654	761
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	13,762	14,343
Recurring | Mortgage-related
Assets:
Trading securities	1,714	4,770
Recurring | Non-mortgage-related securities
Assets:
Trading securities	47,647	47,782
Recurring | Agency
Assets:
Total Fair Value	367	377
Recurring | Other mortgage-related
Assets:
Total Fair Value	168	187
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	47,626	47,764
Total Fair Value	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Derivative assets at fair value	0	0
Total financial assets	47,626	47,764
Liabilities:
Long-term debt:	0	0
Derivative liabilities	0	0
Total financial liabilities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities:
Long-term debt:	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities:
Long-term debt:	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-related
Assets:
Trading securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-mortgage-related securities
Assets:
Trading securities	47,626	47,764
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency
Assets:
Total Fair Value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other mortgage-related
Assets:
Total Fair Value	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	1,707	4,762
Total Fair Value	48	50
Mortgage loans held for investment, net of allowance for loan losses	2,706	2,838
Derivative assets at fair value	353	395
Total financial assets	4,814	8,045
Liabilities:
Long-term debt:	14,029	14,719
Derivative liabilities	3,331	3,327
Total financial liabilities	17,360	18,046
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	381	493
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	13,648	14,226
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	1,686	4,744
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	21	18
Recurring | Significant Other Observable Inputs (Level 2) | Agency
Assets:
Total Fair Value	44	46
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Total Fair Value	4	4
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	26
Total Fair Value	487	514
Mortgage loans held for investment, net of allowance for loan losses	470	477
Derivative assets at fair value	82	90
Total financial assets	1,067	1,107
Liabilities:
Long-term debt:	387	385
Derivative liabilities	13	13
Total financial liabilities	400	398
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	273	268
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	114	117
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	28	26
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Agency
Assets:
Total Fair Value	323	331
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Total Fair Value	$ 164	$ 183