Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator for earnings per common share - basic:
Net income (loss)	$ 12,221	$ (2,814)	$ 16,429	$ (14,792)
Net (income) loss attributable to noncontrolling interest	(31)	6	(41)	30
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net income (loss) attributable to Equity Commonwealth common shareholders	10,193	(4,805)	10,397	(20,753)
Numerator for earnings per common share - diluted:
Net income (loss)	12,221	(2,814)	16,429	(14,792)
Net (income) loss attributable to noncontrolling interest	(31)	6	(41)	30
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Numerator for net income (loss) per share - diluted	$ 10,193	$ (4,805)	$ 10,397	$ (20,753)
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	111,305	122,190	112,341	122,128
Weighted average number of common shares outstanding — diluted (in shares)	112,596	122,190	113,383	122,128
Net income (loss) per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.09	$ (0.04)	$ 0.09	$ (0.17)
Diluted (in dollars per share)	$ 0.09	$ (0.04)	$ 0.09	$ (0.17)
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)	86	262	111	255
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	3,365	3,237	3,365	3,237
RSUs
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	490	0	601
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	0	66	0	93
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	280	223	275	203
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,111	0	875	0
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	180	0	167	0