Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Numerator for earnings per common share - basic and diluted:
Income from continuing operations attributable to Equity Commonwealth common shareholders	$ 13,630	$ 16,725
Preferred distributions	(6,981)	(11,151)
Income from continuing operations attributable to Equity Commonwealth common shareholders	6,649	5,574
Discontinued operations	0	3,723
Numerator for net income per share - basic and diluted	$ 6,649	$ 9,297
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding - basic (in shares)	129,696	118,400
Weighted average common shares outstanding — diluted (in shares)	129,874	118,400
Earnings per share - basic and diluted:
Income from continuing operations (in dollars per share)	$ 0.05	$ 0.05
Discontinued operations (in dollars per share)	$ 0.00	$ 0.03
Net loss per share - basic and diluted (in dollars per share)	$ 0.05us-gaap_EarningsPerShareBasicAndDiluted	$ 0.08us-gaap_EarningsPerShareBasicAndDiluted
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Anti-dilutive securities:
Anti-dilutive securities (in shares)	2,363	7,298

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Income (Loss) from Continuing Operations Available to Common Stockholders

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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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.

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Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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The amount of net income (loss) from continuing operations per each basic and diluted share of common stock or unit when the per share amount is the same for both basic and diluted shares.

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Amount after tax of income (loss) from a discontinued operation attributable to the parent. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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Per basic and diluted share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation and gain (loss) from the disposal of the discontinued operation, when the per share amount is the same.

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

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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