Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Feb. 07, 2022	Jun. 30, 2021
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-07533
Entity Registrant Name	FEDERAL REALTY INVESTMENT TRUST
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	87-3916363
Entity Address, Address Line One	909 Rose Avenue, Suite 200
Entity Address, City or Town	North Bethesda
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20852
City Area Code	301)
Local Phone Number	998-8100
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 9.1
Entity Common Stock, Shares Outstanding		78,616,815
Entity Central Index Key	0000034903
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false
Federal Realty OP LP
Entity Information [Line Items]
Document Type	10-K
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Entity File Number	333-262016-01
Entity Registrant Name	FEDERAL REALTY OP LP
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	52-0782497
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Central Index Key	0001901876
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false
No Trading Symbol Flag
New York Stock Exchange, Inc. | Common Shares Of Beneficial Interest
Entity Information [Line Items]
Title of 12(b) Security	Common Shares of Beneficial Interest
Trading Symbol	FRT
Security Exchange Name	NYSE
New York Stock Exchange, Inc. | Depositary Shares
Entity Information [Line Items]
Title of 12(b) Security	Depositary Shares, each representing 1/1000 of a share
Trading Symbol	FRT-C
Security Exchange Name	NYSE