Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator for earnings per common share - basic:
Net income	$ 25,458	$ 22,751
Net income attributable to noncontrolling interest	(53)	(66)
Preferred distributions	(1,997)	(1,997)
Net income attributable to Equity Commonwealth common shareholders	23,408	20,688
Numerator for earnings per common share - diluted:
Net income	25,458	22,751
Net income attributable to noncontrolling interest	(53)	(66)
Preferred distributions	(1,997)	(1,997)
Numerator for net income per share - diluted	$ 23,408	$ 20,688
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	107,216	109,720
Weighted average number of common shares outstanding — diluted (in shares)	108,224	111,300
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.22	$ 0.19
Diluted (in dollars per share)	$ 0.22	$ 0.19
Anti-dilutive securities:
Preferred shares, dividend percentage	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	129	113
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	4,032	4,032
OP Units and LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	223	324
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	898	1,433
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	110	147