Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator for earnings per common share - basic:
Net income	$ 425,507	$ 210,597
Net income attributable to noncontrolling interest	(748)	(79)
Preferred distributions	(1,997)	(1,997)
Net income attributable to Equity Commonwealth common shareholders	422,762	208,521
Numerator for earnings per common share - diluted:
Net income	425,507	210,597
Net income attributable to noncontrolling interests	(748)	(79)
Numerator for net income per share - diluted	$ 424,759	$ 210,518
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	122,148	121,960
Weighted average number of common shares outstanding — diluted (in shares)	126,605	125,822
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 3.46	$ 1.71
Diluted (in dollars per share)	$ 3.35	$ 1.67
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	89	42
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	73	4
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,524	1,124
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	76	175
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,857	2,563
Anti-dilutive securities:
Preferred shares, dividend percentage	6.50%	6.50%