Debt Instrument, Extension Option Fee, Percent

No definition available.

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Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Number of Extension Options

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Term, Extension Option

+ References

No definition available.

+ Details

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

- Definition

Line Of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

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Namespace Prefix:	eqc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the number of properties secured by mortgage notes.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

Name:	us-gaap_ExtinguishmentOfDebtAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate during the reporting period.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- 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

+ Details

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

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
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+ Details

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

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_LoanFacilitySevenYearTermMember
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Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_RevolvingCreditFacilityandTermLoanMember
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Balance Type:
Period Type:

- Details

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- Details

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- Details

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- Details

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- Details

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Indebtedness - Narrative (Details)			3 Months Ended
Indebtedness - Narrative (Details)	Mar. 31, 2018 USD ($) property	Mar. 07, 2018 USD ($)	Mar. 31, 2018 USD ($) option property	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Senior unsecured notes, net	$ 250,000,000		$ 250,000,000
Loss on early extinguishment of debt			4,867,000	$ 0
Real estate properties, net	867,362,000		867,362,000		$ 1,296,893,000
Mortgage notes payable, net	32,281,000		32,281,000		$ 32,594,000
Loan facility, 5-year term
Debt Instrument [Line Items]
Term loan amount outstanding	200,000,000.0		$ 200,000,000.0
Interest accrual rate			3.03%
Loan facility, 5-year term \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate			1.15%
Loan facility, 5-year term \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			0.90%
Loan facility, 5-year term \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			1.80%
Loan facility, 7-year term
Debt Instrument [Line Items]
Term loan amount outstanding	200,000,000		$ 200,000,000
Interest accrual rate			3.43%
Loan facility, 7-year term \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate			1.55%
Loan facility, 7-year term \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			1.40%
Loan facility, 7-year term \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			2.35%
Revolving credit facility and term loans
Debt Instrument [Line Items]
Additional increases in maximum amount of borrowings available	$ 1,150,000,000.00		$ 1,150,000,000.00
6.65% Senior Unsecured Notes Due 2018
Debt Instrument [Line Items]
Debt redeemed at par		$ 175,000,000
Debt instrument, interest rate		5.75%
Loss on early extinguishment of debt		$ 4,900,000
Mortgage notes
Debt Instrument [Line Items]
Number of real estate properties secured by mortgage \| property	2		2
Real estate properties, net	$ 51,900,000		$ 51,900,000
Mortgage notes payable, net	32,300,000		$ 32,300,000
Revolving credit facility
Debt Instrument [Line Items]
Interest rate payable percentage			2.93%
Amount outstanding	0		$ 0
Revolving credit facility \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage			0.125%
Revolving credit facility \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage			0.30%
Revolving credit facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate			1.05%
Revolving credit facility \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			0.875%
Revolving credit facility \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			1.55%
Revolving credit facility \| Base rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			0.00%
Revolving credit facility \| Base rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			0.55%
Revolving credit facility \| Unsecured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	750,000,000.0		$ 750,000,000.0
Debt instrument, number of extension options \| option			2
Debt instrument, extension option term			6 months
Debt instrument, extension option fee, percent			0.075%
Amount available for borrowing	$ 750,000,000.0		$ 750,000,000.0
Commitment fee percentage	0.20%
Revolving credit facility \| Loan facility, 5-year term
Debt Instrument [Line Items]
Term loan amount outstanding	$ 200,000,000.0		$ 200,000,000.0
Debt instrument, term			5 years
Revolving credit facility \| Loan facility, 7-year term
Debt Instrument [Line Items]
Term loan amount outstanding	200,000,000.0		$ 200,000,000.0
Debt instrument, term			7 years
Letter of credit \| Unsecured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100,000,000.0		$ 100,000,000.0