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		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator for earnings per common share - basic and diluted:
Income from continuing operations	$ 12,616	$ 6,688	$ 26,246	$ 23,413
Preferred distributions	(6,981)	(6,982)	(13,962)	(18,133)
Excess fair value of consideration over carrying value of preferred shares	0	(16,205)	0	(16,205)
Income (loss) from continuing operations attributable to Equity Commonwealth common shareholders	5,635	(16,499)	12,284	(10,925)
Discontinued operations	0	(1,303)	0	2,420
Numerator for net income (loss) per share - basic and diluted	$ 5,635	$ (17,802)	$ 12,284	$ (8,505)
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	129,733	123,812	129,714	121,121
Weighted average common shares outstanding — diluted (in shares)	130,537	123,812	130,205	121,121
Earnings per share - basic and diluted:
Income (loss) from continuing operations (in dollars per share)	$ 0.04	$ (0.13)	$ 0.09	$ (0.09)
Discontinued operations (in dollars per share)	0.00	(0.01)	0.00	0.02
Net income (loss) per share - basic and diluted (in dollars per share)	$ 0.04	$ (0.14)	$ 0.09	$ (0.07)
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Anti-dilutive securities:
Anti-dilutive securities (in shares)	2,363	4,721	2,363	6,017