Related Person Transactions - Narrative (Details) - Two North Riverside Plaza Joint Venture Limited Partnership $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($) option	Mar. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Lease term	5 years
Number of renewal options of lease arrangement \| option	1
Renewal term of lease arrangement	5 years
Tenant improvements	$ 0.7
Lease termination period duration	30 days
Expenses from transactions with related party	$ 0.2	$ 0.2

Lessee Leasing Arrangements, Operating Leases, Termination Period Duration

No definition available.

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Namespace Prefix:	eqc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the information pertaining to lease term of office space, which the entity leases to related parties.

+ References

No definition available.

+ Details

Name:	eqc_RelatedPartyTransactionLeaseTerm
Namespace Prefix:	eqc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of renewal options of lease arrangement entered into with the related party by the entity.

+ References

No definition available.

+ Details

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

- Definition

Represents the renewal term of lease arrangement entered into with the related party by the entity.

+ References

No definition available.

+ Details

Name:	eqc_RelatedPartyTransactionRenewalTermOfLeaseArrangement
Namespace Prefix:	eqc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants.

+ 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(f))
-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

- Details

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Namespace Prefix:
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Period Type: