Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Assets:
Total Fair Value	$ 584	$ 696
Mortgage loans held for investment, net of allowance for loan losses	3,207	3,645
Derivative assets at fair value	149	175
Derivative assets, gross amount offset	(321)	(154)
Liabilities:
Derivative liabilities	225	168
Fannie Mae
Liabilities:
Long-term debt:	833	1,161
Consolidated Trusts
Liabilities:
Long-term debt:	14,210	16,260
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents, including restricted cash equivalents	34,649	32,991
Trading securities	48,634	46,898
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	83,283	79,889
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:
Cash equivalents, including restricted cash equivalents	47,150	54,850
Trading securities	2,630	3,184
Total Fair Value	169	62
Mortgage loans held for investment, net of allowance for loan losses	3,346,542	3,437,979
Derivative assets at fair value	417	300
Total financial assets	3,423,730	3,512,490
Liabilities:
Derivative liabilities	4,452	4,764
Total financial liabilities	3,550,088	3,648,996
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	109,706	122,066
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,421,692	3,511,958
Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents, including restricted cash equivalents	0	0
Trading securities	24	47
Total Fair Value	415	634
Mortgage loans held for investment, net of allowance for loan losses	123,867	171,857
Derivative assets at fair value	53	29
Total financial assets	126,589	174,762
Liabilities:
Derivative liabilities	14	66
Total financial liabilities	884	42,840
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	551	558
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	248	42,150
Recurring
Assets:
Cash equivalents, including restricted cash equivalents	494	0
Trading securities	51,288	50,129
Total Fair Value	584	696
Mortgage loans held for investment, net of allowance for loan losses	3,207	3,645
Derivative assets at fair value	149	175
Derivative assets, gross amount offset	(321)	(154)
Total financial assets	55,722	54,645
Liabilities:
Long-term debt:	15,043	17,421
Derivative liabilities	225	168
Derivative liabilities, gross amount offset	(4,241)	(4,662)
Total financial liabilities	15,268	17,589
Recurring | Fannie Mae
Liabilities:
Long-term debt:	833	1,161
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	14,210	16,260
Recurring | Mortgage-related
Assets:
Trading securities	2,634	3,211
Recurring | Non-mortgage-related securities
Assets:
Trading securities	48,654	46,918
Recurring | Other agency
Assets:
Total Fair Value	376	426
Recurring | Other mortgage-related
Assets:
Total Fair Value	208	270
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents, including restricted cash equivalents	494	0
Trading securities	48,634	46,898
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	49,128	46,898
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	48,634	46,898
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other 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:
Cash equivalents, including restricted cash equivalents	0	0
Trading securities	2,630	3,184
Total Fair Value	169	62
Mortgage loans held for investment, net of allowance for loan losses	2,716	3,102
Derivative assets at fair value	417	300
Total financial assets	5,932	6,648
Liabilities:
Long-term debt:	14,734	17,043
Derivative liabilities	4,452	4,764
Total financial liabilities	19,186	21,807
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	593	919
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	14,141	16,124
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	2,610	3,164
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	20	20
Recurring | Significant Other Observable Inputs (Level 2) | Other agency
Assets:
Total Fair Value	164	55
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Total Fair Value	5	7
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents, including restricted cash equivalents	0	0
Trading securities	24	47
Total Fair Value	415	634
Mortgage loans held for investment, net of allowance for loan losses	491	543
Derivative assets at fair value	53	29
Total financial assets	983	1,253
Liabilities:
Long-term debt:	309	378
Derivative liabilities	14	66
Total financial liabilities	323	444
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	240	242
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	69	136
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	24	47
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Other agency
Assets:
Total Fair Value	212	371
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Total Fair Value	$ 203	$ 263