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Organization - Schedule of Property Level Information by Geographic Region and Property Type, Property Square Feet (Details)
Dec. 31, 2014
sqft
Dec. 31, 2013
sqft
Segment Reporting Information [Line Items]    
Square Feet 42,919,000us-gaap_AreaOfRealEstateProperty 42,915,000us-gaap_AreaOfRealEstateProperty
Consolidated portfolio    
Segment Reporting Information [Line Items]    
Number of Properties 156us-gaap_NumberOfRealEstateProperties
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_ConsolidatedPropertiesMember		  
Number of Buildings 262eqc_NumberOfBuildings
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_ConsolidatedPropertiesMember		  
Square Feet 42,919,000us-gaap_AreaOfRealEstateProperty
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_ConsolidatedPropertiesMember		  
Suburban property    
Segment Reporting Information [Line Items]    
Number of Properties 29us-gaap_NumberOfRealEstateProperties
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= eqc_SuburbanPropertyMember		  
Suburban property | Wholly owned portfolio    
Segment Reporting Information [Line Items]    
Number of Properties 116us-gaap_NumberOfRealEstateProperties
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= eqc_SuburbanPropertyMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember		  
Number of Buildings 209eqc_NumberOfBuildings
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= eqc_SuburbanPropertyMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember		  
Square Feet 21,027,000us-gaap_AreaOfRealEstateProperty
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= eqc_SuburbanPropertyMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember		  
CBD | Wholly owned portfolio    
Segment Reporting Information [Line Items]    
Number of Properties 40us-gaap_NumberOfRealEstateProperties
/ eqc_RealEstateByTypeAxis
= eqc_CentralBusinessDistrictMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember		  
Number of Buildings 53eqc_NumberOfBuildings
/ eqc_RealEstateByTypeAxis
= eqc_CentralBusinessDistrictMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember		  
Square Feet 21,892,000us-gaap_AreaOfRealEstateProperty
/ eqc_RealEstateByTypeAxis
= eqc_CentralBusinessDistrictMember
/ us-gaap_RealEstatePropertiesAxis
= us-gaap_WhollyOwnedPropertiesMember