Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 01, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	0-50231
Entity Tax Identification Number	52-0883107
Entity Registrant Name	FEDERAL NATIONAL MORTGAGE ASSOCIATION FANNIE MAE
Entity Central Index Key	0000310522
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Small Business	false
Entity Common Stock, Shares Outstanding		1,158,087,567
Entity Public Float			$ 2.5
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Address, Address Line One	1100 15th Street, NW
City Area Code	800
Local Phone Number	232-6643
Entity Address, City or Town	Washington,
Entity Address, State or Province	DC
Entity Address, State or Province	20005
Entity Incorporation, State or Country Code	X1
Entity Emerging Growth Company	false
Amendment Flag	false
ICFR Auditor Attestation Flag	true