Selected Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended	12 Months Ended
Jan. 31, 2014	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues		$ 212,808us-gaap_RealEstateRevenueNet	$ 216,595us-gaap_RealEstateRevenueNet	$ 215,194us-gaap_RealEstateRevenueNet	$ 217,260us-gaap_RealEstateRevenueNet	$ 214,028us-gaap_RealEstateRevenueNet	$ 214,201us-gaap_RealEstateRevenueNet	$ 262,188us-gaap_RealEstateRevenueNet	$ 262,612us-gaap_RealEstateRevenueNet	$ 861,857us-gaap_RealEstateRevenueNet	$ 953,029us-gaap_RealEstateRevenueNet	$ 961,087us-gaap_RealEstateRevenueNet
Net income (loss) attributable to Equity Commonwealth common shareholders		$ (165,542)	$ 149,759	$ (17,802)	$ 9,297	$ (16,452)	$ (227,466)	$ 7,738	$ 14,516	$ (24,288)	$ (221,664)	$ (151,958)
Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic and diluted (in dollars per share)		$ (1.28)us-gaap_EarningsPerShareBasicAndDiluted	$ 1.16us-gaap_EarningsPerShareBasicAndDiluted	$ (0.14)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.08us-gaap_EarningsPerShareBasicAndDiluted	$ (0.14)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.92)us-gaap_EarningsPerShareBasicAndDiluted	$ 0.07us-gaap_EarningsPerShareBasicAndDiluted	$ 0.15us-gaap_EarningsPerShareBasicAndDiluted	$ (0.19)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.97)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.81)us-gaap_EarningsPerShareBasicAndDiluted
Common distributions declared (in dollars per share)	$ 0.25	$ 0.00	$ 0.00	$ 0.00	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25

Aggregate dividends declared during the period for each share of common stock outstanding.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

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- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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- Details

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Balance Type:	na
Period Type:	duration

- Definition

The aggregate revenue from real estate operations during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(e))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration