Real Estate Properties - Narrative (Details) ft² in Millions, $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Real Estate [Abstract]
Real estate property improvements	$ 11.6	$ 13.9	$ 56.3
Committed expenditures on leases executed during period	$ 9.8
Square feet of leases committed for expenditures during the period (in sqft) \| ft²	0.1
Committed but unspent tenant related obligations based on existing leases	$ 9.6
COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Deferred collection of rental income	$ 0.3

Represents committed expenditures for operating leases executed during the period.

No definition available.

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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

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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

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Period Type:	instant

- Definition

Amount of decrease in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time from transfer to receivable due to right to consideration becoming unconditional.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130551-203045

+ Details

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Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_RealEstateAbstract
Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 970
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-28(Footnote 4))
-URI http://asc.fasb.org/extlink&oid=120402810&loc=d3e638233-123024

+ Details

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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

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Balance Type:	na
Period Type:	duration

- Details

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