This element represents the number of buildings sold or agreed to be sold by the entity.

No definition available.

Name:	eqc_NumberOfBuildingsSold
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the number of properties sold or agreed to be sold by the entity.

+ References

No definition available.

+ Details

Name:	eqc_NumberOfPropertiesSold
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The element represents the aggregate sales price excluding mortgage debt repayments and closing costs, of real estate properties sold or agreed to be sold by the entity.

+ References

No definition available.

+ Details

Name:	eqc_RealEstateAggregateSalesPrice
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

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSaleOfProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

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

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=eqc_SaleofPropertiesResultinginGainsandLossesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=eqc_SaleofPropertieswithPreviouslyRecordedAssetImpairmentLossesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

Real Estate Properties - Summary of Properties Sold (Details) $ in Thousands	1 Months Ended										12 Months Ended
	Jun. 30, 2014 USD ($) ft² property	Dec. 31, 2013 USD ($) ft² building property	Nov. 30, 2013 USD ($) ft² building property	Oct. 31, 2013 USD ($) ft² building property	Aug. 31, 2013 USD ($) ft² building property	Jun. 30, 2013 USD ($) ft² building property	May. 31, 2013 USD ($) ft² building property	Apr. 30, 2013 USD ($) ft² building property	Mar. 31, 2013 USD ($) ft² building property	Jan. 31, 2013 USD ($) ft² building property	Dec. 31, 2015 USD ($) ft² building	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($) ft² building property	Mar. 31, 2014 ft²
Real Estate Properties [Line Items]
Number of Properties \| property										3
Number of Buildings \| building										18
Square Footage \| ft²												5,600,000		5,641,450
Gain (Loss) on Sale											$ 84,421	$ 0	$ 1,596
Disposed of by Sale
Real Estate Properties [Line Items]
Number of Buildings \| building											135
Square Footage \| ft²											18,939,185
Sales Price											$ 1,999,413
Gain (Loss) on Sale											$ 84,421
Disposed of by Sale \| Sales of properties resulting in gains and losses
Real Estate Properties [Line Items]
Number of Properties \| property						1				3			7
Number of Buildings \| building						1				18			59
Square Footage \| ft²		2,760,235				30,105				1,060,026			2,760,235
Sales Price						$ 1,600				$ 10,250			$ 105,207
Gain (Loss) on Sale						$ 317				$ 1,277			$ (20,566)
Disposed of by Sale \| Sales of properties with previously recorded asset impairment losses:
Real Estate Properties [Line Items]
Number of Properties \| property	14		18	9	1	1	1	1					36
Number of Buildings	43		21	41	3	2	1	1					75
Square Footage \| ft²	2,784,098	5,583,720	2,125,278	1,211,536	129,452	356,045	57,250	618,000					5,583,720
Sales Price	$ 215,900		$ 50,500	$ 43,000	$ 4,100	$ 16,300	$ 4,025	$ 830					$ 143,925
Gain (Loss) on Sale
Disposed of by Sale \| Properties Sold in December \| Sales of properties resulting in gains and losses
Real Estate Properties [Line Items]
Number of Properties \| property		3
Number of Buildings \| building		40
Square Footage \| ft²		1,670,104											1,670,104
Sales Price		$ 89,000
Gain (Loss) on Sale		$ (25,521)
Disposed of by Sale \| Properties Sold in December \| Sales of properties with previously recorded asset impairment losses:
Real Estate Properties [Line Items]
Number of Properties \| property		2
Number of Buildings \| building		2
Square Footage \| ft²		77,394											77,394
Sales Price		$ 2,020
Disposed of by Sale \| Land parcel \| Sales of properties resulting in gains and losses
Real Estate Properties [Line Items]
Number of Properties \| property						0			0
Number of Buildings \| building						0			0
Square Footage \| ft²						0			0
Sales Price						$ 2,551			$ 1,806
Gain (Loss) on Sale						$ 1,765			$ 1,596
Disposed of by Sale \| Properties Sold in October \| Sales of properties with previously recorded asset impairment losses:
Real Estate Properties [Line Items]
Number of Properties \| property				1
Number of Buildings \| building				2
Square Footage \| ft²				199,418
Sales Price				$ 4,000
Disposed of by Sale \| Other Properties \| Sales of properties with previously recorded asset impairment losses:
Real Estate Properties [Line Items]
Number of Properties \| property				1		1
Number of Buildings \| building				1		1
Square Footage \| ft²				665,545		143,802
Sales Price				$ 13,900		$ 5,250