Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator for earnings per common share - basic:
Net (loss) income	$ (2,814)	$ 370	$ (14,792)	$ 453,763
Net loss (income) attributable to noncontrolling interest	6	(1)	30	(803)
Preferred distributions	(1,997)	(1,997)	(5,991)	(5,991)
Net (loss) income attributable to Equity Commonwealth common shareholders	(4,805)	(1,628)	(20,753)	446,969
Numerator for earnings per common share - diluted:
Net (loss) income	(2,814)	370	(14,792)	453,763
Net loss (income) attributable to noncontrolling interests	6	(1)	30	(803)
Preferred distributions	(1,997)	(1,997)	(5,991)	0
Numerator for net (loss) income per share - diluted	$ (4,805)	$ (1,628)	$ (20,753)	$ 452,960
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	122,190	121,673	122,128	121,824
Series D preferred shares; 6.5% cumulative convertible (in shares)	0	0	0	2,857
Weighted average number of common shares outstanding — diluted (in shares)	122,190	121,673	122,128	126,282
Preferred shares, dividend percentage	6.50%	6.50%	6.50%	6.50%
Net (loss) income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ (0.04)	$ (0.01)	$ (0.17)	$ 3.67
Diluted (in dollars per share)	$ (0.04)	$ (0.01)	$ (0.17)	$ 3.59
Anti-dilutive securities(4):
Weighted average number of shares, restricted stock units, unvested (in shares)	262	150	255	159
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	3,237	2,857	3,237	0
RSUs
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	490	1,525	601	0
LTIP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	66	206	93	115
OP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	223	113	203	99
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	0	0	1,525
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	0	0	76