Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets:
Total Fair Value	$ 492	$ 564
Mortgage loans held for investment, net of allowance for loan losses	3,255	3,315
Derivative assets at fair value	280	202
Derivative assets, gross amount offset	(233)	(283)
Liabilities:
Derivative liabilities	132	140
Fannie Mae
Liabilities:
Long-term debt:	451	761
Consolidated Trusts
Liabilities:
Long-term debt:	13,237	14,343
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	60,080	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	98,002	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	1,183	4,762
Total Fair Value	44	50
Mortgage loans held for investment, net of allowance for loan losses	3,645,412	3,571,555
Derivative assets at fair value	422	395
Total financial assets	3,707,136	3,643,669
Liabilities:
Derivative liabilities	2,275	3,327
Total financial liabilities	3,838,578	3,760,502
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	111,915	106,701
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,712,964	3,633,157
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	35	26
Total Fair Value	448	514
Mortgage loans held for investment, net of allowance for loan losses	131,457	130,022
Derivative assets at fair value	91	90
Total financial assets	132,992	133,003
Liabilities:
Derivative liabilities	11	13
Total financial liabilities	980	976
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	630	605
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	278	293
Recurring
Assets:
Trading securities	61,298	52,552
Total Fair Value	492	564
Mortgage loans held for investment, net of allowance for loan losses	3,255	3,315
Derivative assets at fair value	280	202
Derivative assets, gross amount offset	(233)	(283)
Total financial assets	65,325	56,633
Liabilities:
Long-term debt:	13,688	15,104
Derivative liabilities	132	140
Derivative liabilities, gross amount offset	(2,154)	(3,200)
Total financial liabilities	13,820	15,244
Recurring | Fannie Mae
Liabilities:
Long-term debt:	451	761
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	13,237	14,343
Recurring | Mortgage-related
Assets:
Trading securities	1,198	4,770
Recurring | Non-mortgage-related securities
Assets:
Trading securities	60,100	47,782
Recurring | Agency
Assets:
Total Fair Value	352	377
Recurring | Other mortgage-related
Assets:
Total Fair Value	140	187
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	60,080	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	60,080	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	60,080	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	1,183	4,762
Total Fair Value	44	50
Mortgage loans held for investment, net of allowance for loan losses	2,825	2,838
Derivative assets at fair value	422	395
Total financial assets	4,474	8,045
Liabilities:
Long-term debt:	13,294	14,719
Derivative liabilities	2,275	3,327
Total financial liabilities	15,569	18,046
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	167	493
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	13,127	14,226
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	1,163	4,744
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	20	18
Recurring | Significant Other Observable Inputs (Level 2) | Agency
Assets:
Total Fair Value	39	46
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Total Fair Value	5	4
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	35	26
Total Fair Value	448	514
Mortgage loans held for investment, net of allowance for loan losses	430	477
Derivative assets at fair value	91	90
Total financial assets	1,004	1,107
Liabilities:
Long-term debt:	394	385
Derivative liabilities	11	13
Total financial liabilities	405	398
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	284	268
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	110	117
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	35	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	313	331
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Total Fair Value	$ 135	$ 183