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

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Data Type:	xbrli:integerItemType
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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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Period Type:	duration

- Definition

The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82921503&loc=d3e21338-158488

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Period Type:	instant

- Definition

Reflects the net positive or negative amount derived from subtracting from net proceeds of sale the carrying amounts, net of allocated reserves, of notes receivable transferred to a third party in a transaction that qualifies for sales treatment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107425-111719

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Balance Type:	credit
Period Type:	duration

- Definition

The carrying amount of the outstanding balance due under the mortgage loan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The stated interest rate on the mortgage loan receivable or the weighted average interest rate on a group of loans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Balance Type:	na
Period Type:	duration

- Definition

Total amount of new mortgage loans on real estate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 310
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- Definition

The cash inflow from proceeds from sale of receivables arising from the mortgage note on real estate; includes collections on mortgage notes receivable that are not classified as operating cash flows.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
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Other Assets - Narrative (Details)	1 Months Ended			12 Months Ended
Other Assets - Narrative (Details)	Sep. 30, 2017 USD ($)	Jan. 31, 2013 USD ($) building property	Apr. 30, 2012 USD ($)	Dec. 31, 2017 USD ($) building	Dec. 31, 2016 USD ($) building	Dec. 31, 2015 USD ($) building property
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total real estate mortgage receivable included in other assets, carrying value				$ 7,700,000	$ 8,100,000
Number of properties sold or agreed to be sold \| property		3
Number of buildings sold \| building		18
Principal payments received from real estate mortgages receivable				313,000	0	$ 0
Real estate mortgages receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for losses related to real estate mortgages receivable				$ 0	$ 0
Sales of properties resulting in gains and losses \| Properties Sold in December
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage financing related to property sold			$ 400,000
Interest rate on real estate mortgage receivable			6.00%
Disposed of by Sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of properties sold or agreed to be sold \| property						91
Number of buildings sold \| building				37	62	135
Principal payments received from real estate mortgages receivable	$ 300,000
Disposed of by Sale \| Real estate mortgages receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loss on sale of notes receivable				$ 100,000
Disposed of by Sale \| Sales of properties resulting in gains and losses
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage financing related to property sold		$ 7,700,000
Interest rate on real estate mortgage receivable		6.00%