Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator for earnings per common share - basic:
Net income	$ 26,232	$ 12,221	$ 64,819	$ 16,429
Net income attributable to noncontrolling interest	(86)	(31)	(204)	(41)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income attributable to Equity Commonwealth common shareholders	24,149	10,193	58,624	10,397
Numerator for earnings per common share - diluted:
Net income	26,232	12,221	64,819	16,429
Net income attributable to noncontrolling interest	(86)	(31)	(204)	(41)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Numerator for net income per share - diluted	$ 24,149	$ 10,193	$ 58,624	$ 10,397
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	108,931	111,305	109,494	112,341
Weighted average number of common shares outstanding — diluted (in shares)	110,217	112,596	110,916	113,383
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.22	$ 0.09	$ 0.54	$ 0.09
Diluted (in dollars per share)	$ 0.22	$ 0.09	$ 0.53	$ 0.09
Anti-dilutive securities:
Preferred shares, dividend percentage	6.50%	6.50%	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	131	86	125	111
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	4,032	3,365	4,032	3,365
OP Units and LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	361	280	348	275
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,176	1,111	1,299	875
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	110	180	123	167