Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator for earnings per common share - basic:
Net income	$ 452,093	$ 492,866	$ 272,908
Net income attributable to noncontrolling interest	(799)	(186)	(95)
Preferred distributions	(7,988)	(7,988)	(7,988)
Net income attributable to Equity Commonwealth common shareholders	443,306	484,692	264,825
Numerator for earnings per common share - diluted:
Net income	452,093	492,866	272,908
Net income attributable to noncontrolling interest	(799)	(186)	(95)
Preferred distributions	0	0	(7,988)
Numerator for net income per share - diluted	$ 451,294	$ 492,680	$ 264,825
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	121,786	122,091	122,314
Weighted average common shares outstanding — diluted (in shares)	126,606	126,260	123,385
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 3.64	$ 3.97	$ 2.17
Diluted (in dollars per share)	$ 3.56	$ 3.90	$ 2.15
Anti-dilutive securities:
Preferred shares, dividend yield	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	157	210	308
Series D Preferred Stock
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6.50% cumulative convertible (in shares)	3,237	2,857	0
Series D Preferred Stock
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	0	2,563
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	119	33	43
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	102	14	1
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,508	1,138	956
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	75	174	115