Segment Information - Narrative (Details) $ in Thousands	9 Months Ended
Sep. 30, 2015 USD ($) property building segment suburban_property	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	2
Real estate properties, net	$ 3,088,158	$ 4,697,998
CBD Properties
Segment Reporting Information [Line Items]
Number of real estate properties \| property	24
Number of buildings \| building	31
Real estate properties, net	$ 2,027,300
Suburban Properties
Segment Reporting Information [Line Items]
Number of real estate properties \| suburban_property	43
Number of buildings \| building	101
Real estate properties, net	$ 1,060,900
Minimum
Segment Reporting Information [Line Items]
Percentage of CBD and suburban properties that are office properties	90.00%

The number of buildings owned as of the balance sheet date.

No definition available.

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

- Definition

Represents the percentage of real estate properties considered as office properties.

+ References

No definition available.

+ Details

Name:	eqc_PercentageOfRealEstatePropertiesConsideredAsOfficeProperties
Namespace Prefix:	eqc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=eqc_CentralBusinessDistrictOfficeSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=eqc_SuburbanOfficeSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: