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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator for earnings per common share - basic:
Net income	$ 33,224	$ 86,388	$ 51,235	$ 220,634
Net income attributable to noncontrolling interest	(12)	0	(18)	0
Preferred distributions	(1,997)	(1,997)	(5,991)	(15,959)
Excess fair value of consideration paid over carrying value of preferred shares	0	0	0	(9,609)
Net income attributable to Equity Commonwealth common shareholders	31,215	84,391	45,226	195,066
Numerator for earnings per common share - diluted:
Net income	33,224	86,388	51,235	220,634
Preferred distributions	(1,997)	(1,997)	(5,991)	(15,959)
Excess fair value of consideration paid over carrying value of preferred shares	0	0	0	(9,609)
Numerator for net income per share - diluted	$ 31,227	$ 84,391	$ 45,244	$ 195,066
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	124,089	125,533	124,068	125,627
Weighted average number of common shares outstanding — diluted (in shares)	125,175	126,568	125,194	127,009
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.25	$ 0.67	$ 0.36	$ 1.55
Diluted (in dollars per share)	$ 0.25	$ 0.67	$ 0.36	$ 1.54
Series D
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,363	2,363	2,363	2,363
Preferred shares, dividend yield	6.50%	6.50%	6.50%	6.50%
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	983	1,035	1,023	1,382
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	103	0	103	0