Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator for earnings per common share - basic:
Income from continuing operations	$ 30,466	$ 156,523	$ 56,712	$ 179,936
Preferred distributions	(6,981)	(6,981)	(20,943)	(25,114)
Excess fair value of consideration over carrying value of preferred shares	0	0	0	(16,205)
Income from continuing operations attributable to Equity Commonwealth common shareholders	23,485	149,542	35,769	138,617
Discontinued operations	0	217	0	2,637
Net income attributable to Equity Commonwealth common shareholders	23,485	149,759	35,769	141,254
Net Income (Loss) Attributable to Parent, Diluted [Abstract]
Income from continuing operations attributable to Equity Commonwealth common shareholders	23,485	149,542	35,769	138,617
Addition of Series D preferred distributions	(6,981)	(6,981)	(20,943)	(25,114)
Diluted income from continuing operations attributable to Equity Commonwealth common shareholders	23,485	151,540	35,769	138,617
Discontinued operations	0	217	0	2,637
Numerator for net income per share - diluted	$ 23,485	$ 151,757	$ 35,769	$ 141,254
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	128,739	128,880	129,386	123,736
Weighted average common shares outstanding — diluted (in shares)	129,878	131,243	130,093	123,736
Earnings per share - basic:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)	$ 0.18	$ 1.16	$ 0.28	$ 1.12
Discontinued operations (in dollars per share)	0.00	0.00	0.00	0.02
Net income (in dollars per share)	0.18	1.16	0.28	1.14
Earnings per share - diluted:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)	0.18	1.16	0.27	1.12
Discontinued operations (in dollars per share)	0.00	0.00	0.00	0.02
Net income (in dollars per share)	$ 0.18	$ 1.16	$ 0.27	$ 1.14
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Anti-dilutive securities:
Anti-dilutive securities (in shares)	2,363	0	2,363	4,786
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Numerator for earnings per common share - basic:
Preferred distributions	$ 0	$ 1,998	$ 0	$ 0
Net Income (Loss) Attributable to Parent, Diluted [Abstract]
Addition of Series D preferred distributions	$ 0	$ 1,998	$ 0	$ 0