Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Assets:
Total Fair Value	$ 646	$ 696
Mortgage loans held for investment, net of allowance for loan losses	3,433	3,645
Derivative assets at fair value	145	175
Derivative assets, gross amount offset	(218)	(154)
Liabilities:
Derivative liabilities	297	168
Fannie Mae
Liabilities:
Long-term debt:	884	1,161
Consolidated Trusts
Liabilities:
Long-term debt:	15,172	16,260
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	48,207	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	84,442	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:
Trading securities	2,350	3,184
Total Fair Value	55	62
Mortgage loans held for investment, net of allowance for loan losses	3,455,655	3,437,979
Derivative assets at fair value	281	300
Total financial assets	3,538,105	3,512,490
Liabilities:
Derivative liabilities	4,454	4,764
Total financial liabilities	3,674,709	3,648,996
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	121,057	122,066
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,533,579	3,511,958
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	47
Total Fair Value	591	634
Mortgage loans held for investment, net of allowance for loan losses	166,079	171,857
Derivative assets at fair value	82	29
Total financial assets	167,437	174,762
Liabilities:
Derivative liabilities	70	66
Total financial liabilities	40,276	42,840
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	574	558
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	39,567	42,150
Recurring
Assets:
Trading securities	50,585	50,129
Total Fair Value	646	696
Mortgage loans held for investment, net of allowance for loan losses	3,433	3,645
Derivative assets at fair value	145	175
Derivative assets, gross amount offset	(218)	(154)
Total financial assets	54,809	54,645
Liabilities:
Long-term debt:	16,056	17,421
Derivative liabilities	297	168
Derivative liabilities, gross amount offset	(4,227)	(4,662)
Total financial liabilities	16,353	17,589
Recurring | Fannie Mae
Liabilities:
Long-term debt:	884	1,161
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	15,172	16,260
Recurring | Mortgage-related
Assets:
Trading securities	2,358	3,211
Recurring | Non-mortgage-related securities
Assets:
Trading securities	48,227	46,918
Recurring | Other agency
Assets:
Total Fair Value	405	426
Recurring | Other mortgage-related
Assets:
Total Fair Value	241	270
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	48,207	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	48,207	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	48,207	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:
Trading securities	2,350	3,184
Total Fair Value	55	62
Mortgage loans held for investment, net of allowance for loan losses	2,923	3,102
Derivative assets at fair value	281	300
Total financial assets	5,609	6,648
Liabilities:
Long-term debt:	15,675	17,043
Derivative liabilities	4,454	4,764
Total financial liabilities	20,129	21,807
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	628	919
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	15,047	16,124
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	2,330	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	49	55
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Total Fair Value	6	7
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	47
Total Fair Value	591	634
Mortgage loans held for investment, net of allowance for loan losses	510	543
Derivative assets at fair value	82	29
Total financial assets	1,211	1,253
Liabilities:
Long-term debt:	381	378
Derivative liabilities	70	66
Total financial liabilities	451	444
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	256	242
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	125	136
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	28	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	356	371
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Total Fair Value	$ 235	$ 263