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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator for earnings per common share - basic:
Net income	$ 32,777	$ 33,224	$ 257,486	$ 51,235
Net income attributable to noncontrolling interest	(13)	(12)	(90)	(18)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income attributable to Equity Commonwealth common shareholders	30,767	31,215	251,405	45,226
Numerator for earnings per common share - diluted:
Net income	32,777	33,224	257,486	51,235
Net income attributable to noncontrolling interests	(13)	0	(90)	0
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Numerator for net income per share - diluted	$ 30,767	$ 31,227	$ 251,405	$ 45,244
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	121,845	124,089	122,504	124,068
Weighted average number of common shares outstanding — diluted (in shares)	122,851	125,175	123,389	125,194
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.25	$ 0.25	$ 2.05	$ 0.36
Diluted (in dollars per share)	$ 0.25	$ 0.25	$ 2.04	$ 0.36
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	2,363	2,363	2,363	2,363
Preferred shares, dividend yield			6.50%	6.50%
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	45	0	43	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	1	0	1	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	894	983	795	1,023
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	112	103	90	103