Name:	eqc_NumberOfRealEstatePropertiesHeldForSale
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of long-lived held and used properties held for use.

+ References

No definition available.

+ Details

Name:	eqc_NumberOfRealEstatePropertiesHeldForUse
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents committed expenditures for operating leases executed during the period.

+ References

No definition available.

+ Details

Name:	eqc_OperatingLeasesCommittedExpendituresOnLeasesExecutedInPeriod
Namespace Prefix:	eqc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the area of leased property related to operating leases executed during the period.

+ References

No definition available.

+ Details

Name:	eqc_OperatingLeasesCommittedExpendituresOnLeasesExecutedInPeriodAreaOfLeasedProperty
Namespace Prefix:	eqc_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents committed but unspent tenant related obligations based on executed operating leases as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	eqc_OperatingLeasesCommittedExpendituresOnLeasesExecutedInPeriodCommittedButUnspentTenantRelatedObligations
Namespace Prefix:	eqc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the reversal of amount of write-down of assets recognized in the income statement. Includes, but is not limited to, reversal of losses from tangible assets, intangible assets and goodwill.

+ References

No definition available.

+ Details

Name:	eqc_ReversalOfLossOnAssetImpairment
Namespace Prefix:	eqc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of separate real estate development properties located on land subject to ground leases.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfPropertiesSubjectToGroundLeases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of increase in real estate investments related to improvements for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

+ Details

Name:	us-gaap_RealEstateImprovements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RealEstatePropertiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=eqc_CentralBusinessDistrictAndSuburbanPropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=eqc_CentralBusinessDistrictMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=eqc_SuburbanPropertyMember
Namespace Prefix:
Data Type:	na
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Period Type:

Real Estate Properties - Narrative (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2014 sqft property	Dec. 31, 2013 sqft	Dec. 31, 2012
Real Estate Properties [Line Items]
Real estate property improvements		$ 91.6us-gaap_RealEstateImprovements	$ 122.9us-gaap_RealEstateImprovements	$ 134.6us-gaap_RealEstateImprovements
Committed expenditures on leases executed during period		88.6
Square feet of leases committed for expenditures during the period		4,792,000
Committed but unspent tenant related obligations based on existing leases		99.9
Square feet of property held for sale			8,425,548eqc_AreaOfRealEstatePropertyHeldForSale
Property square feet		42,919,000us-gaap_AreaOfRealEstateProperty	42,915,000us-gaap_AreaOfRealEstateProperty
Reversal of previously recorded impairment losses	4.8eqc_ReversalOfLossOnAssetImpairment
Number of real estate properties subject to ground lease		1
Ground lease expenses		$ 1.8us-gaap_LeaseAndRentalExpense	$ 1.9us-gaap_LeaseAndRentalExpense	$ 1.8us-gaap_LeaseAndRentalExpense
CBD
Real Estate Properties [Line Items]
Number of real estate properties held for sale			3
Number of buildings held for sale			4
Number of real estate properties held for use	2
Number of buildings held for use	2
Suburban property
Real Estate Properties [Line Items]
Number of real estate properties held for sale			42
Number of buildings held for sale			106
Number of real estate properties held for use	29
Number of buildings held for use	65
CBD and suburban properties
Real Estate Properties [Line Items]
Property square feet	5,641,450	5,641,450