Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator for earnings per common share - basic:
Net (loss) income	$ (16,429)	$ 452,093	$ 492,866
Net loss (income) attributable to noncontrolling interest	33	(799)	(186)
Preferred distributions	(7,988)	(7,988)	(7,988)
Net (loss) income attributable to Equity Commonwealth common shareholders	(24,384)	443,306	484,692
Numerator for earnings per common share - diluted:
Net income	(16,429)	452,093	492,866
Net loss (income) attributable to noncontrolling interest	33	(799)	(186)
Preferred distributions	(7,988)	0	0
Numerator for net (loss) income per share - diluted	$ (24,384)	$ 451,294	$ 492,680
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	121,411	121,786	122,091
Weighted average common shares outstanding — diluted (in shares)	121,411	126,606	126,260
Net (loss) income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ (0.20)	$ 3.64	$ 3.97
Diluted (in dollars per share)	$ (0.20)	$ 3.56	$ 3.90
Anti-dilutive securities(4):
Preferred shares, dividend yield	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	256	157	210
Series D Preferred Stock
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6.50% cumulative convertible (in shares)	0	3,237	2,857
Anti-dilutive securities(4):
Preferred shares, dividend yield	6.50%	6.50%	6.50%
Series D Preferred Stock
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	3,237	0	0
RSUs
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	549	0	0
LTIP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	84	119	33
OP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	208	102	14
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	1,508	1,138
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	75	174