Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator for earnings per common share - basic:
Net (loss) income	$ (10,038)	$ 425,507
Net loss (income) attributable to noncontrolling interest	20	(748)
Preferred distributions	(1,997)	(1,997)
Net (loss) income attributable to Equity Commonwealth common shareholders	(12,015)	422,762
Numerator for earnings per common share - diluted:
Net (loss) income	(10,038)	425,507
Net income attributable to noncontrolling interests	20	(748)
Preferred distributions	(1,997)	0
Numerator for net (loss) income per share - diluted	$ (12,015)	$ 424,759
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	122,002	122,148
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	0	2,857
Weighted average number of common shares outstanding — diluted (in shares)	122,002	126,605
Preferred shares, dividend percentage	6.50%
Net (loss) income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ (0.10)	$ 3.46
Diluted (in dollars per share)	$ (0.10)	$ 3.35
Anti-dilutive securities:
Weighted average number of shares, restricted stock units, unvested (in shares)	236	177
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	3,237	0
RSUs
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	848	0
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	149	89
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	170	73
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	1,524
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	76