Fair Value - Recurring Changes in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Assets:
Total Fair Value	$ 564	$ 696
Mortgage loans	$ 3,315	$ 3,645
Derivative Asset Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Derivative assets	Derivative assets
Derivative assets at fair value	$ 202	$ 175
Derivative asset, gross amount offset	$ (283)	$ (154)
Liabilities:
Derivative Liability Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Derivative liabilities	Derivative liabilities
Derivative liabilities at fair value	$ 140	$ 168
Fannie Mae
Liabilities:
Long-term debt	761	1,161
Consolidated Trusts
Liabilities:
Long-term debt	14,343	16,260
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents, including restricted cash and cash equivalents	33,981	32,991
Trading securities	47,764	46,898
Total Fair Value	0	0
Mortgage loans	0	0
Derivative assets at fair value	0	0
Total assets at fair value	81,745	79,889
Liabilities:
Derivative liabilities at fair value	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 and cash equivalents, including restricted cash and cash equivalents	34,725	54,850
Trading securities	4,762	3,184
Total Fair Value	50	62
Mortgage loans	3,571,555	3,437,979
Derivative assets at fair value	395	300
Total assets at fair value	3,643,669	3,512,490
Liabilities:
Derivative liabilities at fair value	3,327	4,764
Total financial liabilities	3,760,502	3,648,996
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	106,701	122,066
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	3,633,157	3,511,958
Significant Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents, including restricted cash and cash equivalents	0	0
Trading securities	26	47
Total Fair Value	514	634
Mortgage loans	130,022	171,857
Derivative assets at fair value	90	29
Total assets at fair value	133,003	174,762
Liabilities:
Derivative liabilities at fair value	13	66
Total financial liabilities	976	42,840
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	605	558
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	293	42,150
Recurring
Assets:
Trading securities	52,552	50,129
Total Fair Value	564	696
Mortgage loans	3,315	3,645
Derivative assets at fair value	202	175
Derivative asset, gross amount offset	(283)	(154)
Total assets at fair value	56,633	54,645
Liabilities:
Long-term debt	15,104	17,421
Derivative liabilities at fair value	140	168
Derivative liabilities, gross amount offset	(3,200)	(4,662)
Total financial liabilities	15,244	17,589
Recurring | Mortgage-related
Assets:
Trading securities	4,770	3,211
Recurring | Non-mortgage-related securities
Assets:
Trading securities	47,782	46,918
Recurring | Agency securities
Assets:
Total Fair Value	377	426
Recurring | Other
Assets:
Total Fair Value	187	270
Recurring | Fannie Mae
Liabilities:
Long-term debt	761	1,161
Recurring | Consolidated Trusts
Liabilities:
Long-term debt	14,343	16,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	47,764	46,898
Total Fair Value	0	0
Mortgage loans	0	0
Derivative assets at fair value	0	0
Total assets at fair value	47,764	46,898
Liabilities:
Long-term debt	0	0
Derivative liabilities at fair value	0	0
Total financial liabilities	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	47,764	46,898
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency securities
Assets:
Total Fair Value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Total Fair Value	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 | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	4,762	3,184
Total Fair Value	50	62
Mortgage loans	2,838	3,102
Derivative assets at fair value	395	300
Total assets at fair value	8,045	6,648
Liabilities:
Long-term debt	14,719	17,043
Derivative liabilities at fair value	3,327	4,764
Total financial liabilities	18,046	21,807
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	4,744	3,164
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	18	20
Recurring | Significant Other Observable Inputs (Level 2) | Agency securities
Assets:
Total Fair Value	46	55
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets:
Total Fair Value	4	7
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	493	919
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	14,226	16,124
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	26	47
Total Fair Value	514	634
Mortgage loans	477	543
Derivative assets at fair value	90	29
Total assets at fair value	1,107	1,253
Liabilities:
Long-term debt	385	378
Derivative liabilities at fair value	13	66
Total financial liabilities	398	444
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	26	47
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Agency securities
Assets:
Total Fair Value	331	371
Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets:
Total Fair Value	183	263
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	268	242
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	$ 117	$ 136