Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator for earnings per common share - basic:
Net income	$ 22,751	$ 1,305
Net income attributable to noncontrolling interest	(66)	(3)
Preferred distributions	(1,997)	(1,997)
Net income (loss) attributable to Equity Commonwealth common shareholders	20,688	(695)
Numerator for earnings per common share - diluted:
Net income	22,751	1,305
Net income attributable to noncontrolling interest	(66)	(3)
Preferred distributions	(1,997)	(1,997)
Numerator for net income (loss) per share - diluted	$ 20,688	$ (695)
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	109,720	113,740
Weighted average number of common shares outstanding — diluted (in shares)	111,300	113,740
Net income (loss) per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.19	$ (0.01)
Diluted (in dollars per share)	$ 0.19	$ (0.01)
Anti-dilutive securities:
Preferred shares, dividend percentage	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	113	162
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	4,032	3,237
RSUs
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	357
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	148
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	324	223
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,433	0
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	147	0