Segment Information - Narrative (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014 segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of reportable segments	2us-gaap_NumberOfReportableSegments
Real estate properties, net	$ 4,697,998us-gaap_RealEstateInvestmentPropertyNet	$ 4,642,106us-gaap_RealEstateInvestmentPropertyNet
Rental income	691,699	763,262	774,600
CBD properties
Segment Reporting Information [Line Items]
Number of properties	40
Number of buildings	53
Real estate properties, net	2,966,926	3,001,397
Rental income	436,426	439,716	423,453
Suburban properties
Segment Reporting Information [Line Items]
Number of properties	116
Number of buildings	209
Real estate properties, net	1,731,072	1,640,709
Rental income	255,273	323,546	351,147
Minimum
Segment Reporting Information [Line Items]
Percentage of CBD and suburban properties that are office properties	90.00%
Australia
Segment Reporting Information [Line Items]
Real estate properties, net	252,000	285,100	362,000
Rental income	$ 30,100	$ 33,100	$ 34,700

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

The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=7501430&loc=d3e39896-112707

+ Details

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

- Details

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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
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