Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator for earnings per common share - basic:
Net income (loss)	$ 1,305	$ (10,038)
Net (income) loss attributable to noncontrolling interest	(3)	20
Preferred distributions	(1,997)	(1,997)
Net loss attributable to Equity Commonwealth common shareholders	(695)	(12,015)
Numerator for earnings per common share - diluted:
Net income (loss)	1,305	(10,038)
Net (income) loss attributable to noncontrolling interest	(3)	20
Preferred distributions	(1,997)	(1,997)
Numerator for net loss per share - diluted	$ (695)	$ (12,015)
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	113,740	122,002
Weighted average number of common shares outstanding — diluted (in shares)	113,740	122,002
Preferred shares, dividend percentage	6.50%	6.50%
Net loss per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ (0.01)	$ (0.10)
Diluted (in dollars per share)	$ (0.01)	$ (0.10)
Anti-dilutive securities(2):
Weighted average number of shares, restricted stock units, unvested (in shares)	162	236
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities(2):
Effect of anti-dilutive securities (in shares)	3,237	3,237
RSUs
Anti-dilutive securities(2):
Effect of anti-dilutive securities (in shares)	357	848
LTIP Units
Anti-dilutive securities(2):
Effect of anti-dilutive securities (in shares)	148	149
OP Units
Anti-dilutive securities(2):
Effect of anti-dilutive securities (in shares)	223	170