Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Jul. 26, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Entity File Number	1-9317
Entity Registrant Name	EQUITY COMMONWEALTH
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	04-6558834
Entity Address, Address Line One	Two North Riverside Plaza, Suite 2100
Entity Address, City or Town	Chicago
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60606
City Area Code	(312)
Local Phone Number	646-2800
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
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		121,940,355
Entity Central Index Key	0000803649
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Common Shares of Beneficial Interest
Document Information [Line Items]
Title of 12(b) Security	Common Shares of Beneficial Interest
Trading Symbol	EQC
Security Exchange Name	NYSE
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Document Information [Line Items]
Title of 12(b) Security	6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Trading Symbol	EQCpD
Security Exchange Name	NYSE