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		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Numerator for earnings per common share - basic:
Net income	$ 242,377	$ 37,047	$ 452,974	$ 224,709
Net income attributable to noncontrolling interest	(91)	(14)	(170)	(77)
Preferred distributions	(1,997)	(1,997)	(3,994)	(3,994)
Net income attributable to Equity Commonwealth common shareholders	240,289	35,036	448,810	220,638
Numerator for earnings per common share - diluted:
Net income	242,377	37,047	452,974	224,709
Net income attributable to noncontrolling interests	(91)	(14)	(170)	(77)
Distributed Earnings	0	(1,997)	0	0
Numerator for net income per share - diluted	$ 242,286	$ 35,036	$ 452,804	$ 224,632
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	122,122	121,822	122,041	122,839
Weighted average number of common shares outstanding — diluted (in shares)	125,862	122,649	125,841	126,027
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 1.97	$ 0.29	$ 3.68	$ 1.80
Diluted (in dollars per share)	$ 1.93	$ 0.29	$ 3.60	$ 1.78
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	2,363	0	0
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	32	42	34	42
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	14	1	12	1
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,005	762	1,064	746
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	172	65	173	79
Series D
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,563	0	2,563	2,363
Anti-dilutive securities:
Preferred shares, dividend yield	6.50%	6.50%	6.50%	6.50%	6.50%