Debt Instrument, Debt Default, Change of Control

No definition available.

Name:	eqc_DebtInstrumentDebtDefaultChangeofControl
Namespace Prefix:	eqc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Extension Option Fee, Percent

+ References

No definition available.

+ Details

Name:	eqc_DebtInstrumentExtensionOptionFeePercent
Namespace Prefix:	eqc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Number of Extension Options

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Term, Extension Option

+ References

No definition available.

+ Details

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

- Definition

This element represents the maximum borrowing capacity under the credit facility as a result of an increase in borrowing capacity available to the entity.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of buildings secured by mortgage notes.

+ References

No definition available.

+ Details

Name:	eqc_NumberOfBuildingsSecuredByMortgage
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of properties secured by mortgage notes.

+ References

No definition available.

+ Details

Name:	eqc_NumberOfRealEstatePropertiesSecuredByMortgage
Namespace Prefix:	eqc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The period of time the stated maturity date of the revolving credit facility may be extended under certain conditions and fee payment.

+ References

No definition available.

+ Details

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

- 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 reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

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

- Details

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

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- 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 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: 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=6850294&loc=d3e12317-112629

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 50

-Section 40

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12355-112629

+ Details

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

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityAmountOutstanding
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

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

- Definition

The effective interest rate during the reporting period.

+ 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 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateDuringPeriod
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

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
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

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
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

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(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_LoanFacilityFiveYearTermMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_LoanFacilitySevenYearTermMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_MortgageNotesPayableEncumbering3920ArkwrightRoadMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_MortgageNotesPayableEncumbering6200GlennCarlsonDriveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_SeniorNotesFivePointSevenFivePercentDue2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_SeniorNotesSevenPointFiveZeroPercentDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_SeniorNotesSixPointFourZeroPercentDue2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_UnsecuredRevolvingCreditFacilityAndUnsecuredTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_UnsecuredRevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=eqc_UnsecuredTermLoanDueDecember2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExtinguishmentOfDebtAxis=us-gaap_MortgagesMember
Namespace Prefix:
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Balance Type:
Period Type:

Indebtedness - Narrative (Details) (USD $)	1 Months Ended	12 Months Ended			0 Months Ended			1 Months Ended	0 Months Ended			1 Months Ended	0 Months Ended
Indebtedness - Narrative (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 29, 2015 option	Dec. 05, 2014	Oct. 31, 2014	Dec. 31, 2013	Jun. 27, 2014	Nov. 17, 2014	Sep. 15, 2014	Oct. 31, 2013	Aug. 01, 2014	Mar. 11, 2014	Jun. 06, 2014	Jun. 05, 2014	Jul. 11, 2014
Debt Instrument [Line Items]
Gain (loss) on early extinguishment of debt	$ (60,000,000)	$ 4,909,000	$ (60,052,000)	$ (287,000)
Outstanding principal	670,295,000
Purchase price	726,151,000us-gaap_DebtInstrumentRepurchaseAmount
Real estate properties, net		4,697,998,000us-gaap_RealEstateInvestmentPropertyNet	4,642,106,000us-gaap_RealEstateInvestmentPropertyNet					4,642,106,000us-gaap_RealEstateInvestmentPropertyNet
Revolving Credit Facility \| Subsequent Event
Debt Instrument [Line Items]
Number of extensions					2
Extension period					6 months
Extension fee, percent					0.075%
Unsecured Revolving Credit Facility, due October 2015
Debt Instrument [Line Items]
Maximum borrowing capacity		750,000,000
Maturity date extension period available		1 year
Basis spread on variable rate		1.50%	1.50%
Commitment fee percentage		0.35%
Interest rate payable percent		1.70%	1.70%
Amount outstanding		0
Available borrowing capacity		750,000,000
Interest rate stated percentage		1.669%
Unsecured Term Loan, due December 2016
Debt Instrument [Line Items]
Basis spread on variable rate		1.85%	1.85%
Interest rate payable percent		2.00%	2.00%
Amount outstanding						400,000,000
Increased maximum borrowing capacity available under specified circumstances		1,000,000,000
Variable rate basis		LIBOR
Repayments of debt						100,000,000
Interest rate stated percentage		2.019%
Unsecured Revolving Credit Facility and Unsecured Term Loan
Debt Instrument [Line Items]
Change of control event of default															35.00%	25.00%
Unsecured Revolving Credit Facility \| Revolving Credit Facility \| Subsequent Event
Debt Instrument [Line Items]
Maximum borrowing capacity					750,000,000.0
Loan Facility, 5 Year Term \| Revolving Credit Facility \| Subsequent Event
Debt Instrument [Line Items]
Maximum borrowing capacity					200,000,000.0
Term of loan					5 years
Loan Facility, 7 Year Term \| Revolving Credit Facility \| Subsequent Event
Debt Instrument [Line Items]
Maximum borrowing capacity					200,000,000.0
Term of loan					7 years
Mortgages
Debt Instrument [Line Items]
Number of real estate properties secured by mortgage		10
Number of buildings secured by mortgage		14
Real estate properties, net		705,300,000
Mortgages \| 225 Water Street, Jacksonville, FL
Debt Instrument [Line Items]
Interest rate stated percentage																	10.03%
Senior Notes, due 2019 at 7.50%
Debt Instrument [Line Items]
Interest rate stated percentage		0.00%
Senior Notes, due 2015 at 6.40%
Debt Instrument [Line Items]
Interest rate stated percentage		0.00%
Senior Notes, due 2014 at 5.75%
Debt Instrument [Line Items]
Outstanding principal	145,612,000
Purchase price	148,746,000
Mortgage Notes Payable Encumbering 6200 Glenn Carlson Drive
Debt Instrument [Line Items]
Repayments of debt							7,800,000
Interest rate stated percentage		0.00%
Mortgage Notes Payable Encumbering 3920 Arkwright Road
Debt Instrument [Line Items]
Interest rate stated percentage		0.00%
Mortgages
Debt Instrument [Line Items]
Prepayments of debt								7,100,000
Interest rate stated percentage			5.76%					5.76%
Gain (loss) on early extinguishment of debt								(1,000,000)
Mortgages \| Mortgages
Debt Instrument [Line Items]
Gain (loss) on early extinguishment of debt									(3,300,000)
Mortgages \| Senior Notes, due 2019 at 7.50%
Debt Instrument [Line Items]
Prepayments of debt										125,000,000
Interest rate stated percentage										7.50%
Gain (loss) on early extinguishment of debt										(1,800,000)
Mortgages \| Senior Notes, due 2015 at 6.40%
Debt Instrument [Line Items]
Prepayments of debt											33,400,000
Interest rate stated percentage											6.40%
Mortgages \| Senior Notes, due 2014 at 5.75%
Debt Instrument [Line Items]
Prepayments of debt												99,000,000
Interest rate stated percentage												5.75%
Mortgages \| Mortgage Notes Payable, due 2017 at 5.68%
Debt Instrument [Line Items]
Prepayments of debt													265,000,000
Interest rate stated percentage													5.68%
Gain (loss) on early extinguishment of debt													6,700,000
Mortgages \| Mortgage Notes Payable, due 2023 at 6.14%
Debt Instrument [Line Items]
Prepayments of debt									11,200,000
Interest rate stated percentage									6.14%
Mortgages \| Mortgage Notes Payable, due 2015 at 5.78%
Debt Instrument [Line Items]
Prepayments of debt									8,500,000
Interest rate stated percentage									5.78%
Mortgages \| Mortgage Notes Payable Encumbering 3920 Arkwright Road
Debt Instrument [Line Items]
Prepayments of debt														$ 12,000,000
Interest rate stated percentage														4.95%