Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator for earnings per common share - basic:
Net income	$ 15,836	$ 2,903	$ 38,587	$ 4,208
Net income attributable to noncontrolling interest	(52)	(7)	(118)	(10)
Preferred distributions	(1,997)	(1,997)	(3,994)	(3,994)
Net income attributable to Equity Commonwealth common shareholders	13,787	899	34,475	204
Numerator for earnings per common share - diluted:
Net income	15,836	2,903	38,587	4,208
Net income attributable to noncontrolling interest	(52)	(7)	(118)	(10)
Preferred distributions	(1,997)	(1,997)	(3,994)	(3,994)
Numerator for net income per share - diluted	$ 13,787	$ 899	$ 34,475	$ 204
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	109,839	112,005	109,779	112,868
Weighted average number of common shares outstanding — diluted (in shares)	111,237	113,380	111,269	113,785
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.13	$ 0.01	$ 0.31	$ 0.00
Diluted (in dollars per share)	$ 0.12	$ 0.01	$ 0.31	$ 0.00
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	122	124
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	4,032	3,237
OP Units and LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	357	277	341	272
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,288	1,158	1,361	757
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	110	217	129	160