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		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Numerator for earnings per common share - basic:
Net income	$ 23,896	$ 32,777	$ 476,870	$ 257,486
Net income attributable to noncontrolling interest	(10)	(13)	(180)	(90)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income attributable to Equity Commonwealth common shareholders	21,889	30,767	470,699	251,405
Numerator for earnings per common share - diluted:
Net income	23,896	32,777	476,870	257,486
Net income attributable to noncontrolling interests	(10)	(13)	(180)	(90)
Distributed Earnings	(1,997)	(1,997)	0	(5,991)
Numerator for net income per share - diluted	$ 21,889	$ 30,767	$ 476,690	$ 251,405
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	122,140	121,845	122,075	122,504
Weighted average number of common shares outstanding — diluted (in shares)	123,564	122,851	125,938	123,389
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.18	$ 0.25	$ 3.86	$ 2.05
Diluted (in dollars per share)	$ 0.18	$ 0.25	$ 3.79	$ 2.04
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	2,563	2,363	0	2,363
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	32	45	33	43
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	17	1	13	1
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,240	894	1,123	795
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	184	112	177	90
Series D
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	0	0	2,563	0
Anti-dilutive securities:
Preferred shares, dividend yield	6.50%	6.50%	6.50%	6.50%	6.50%