Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Numerator for earnings per common share - basic:
Net income	$ 27,886	$ 242,377	$ 453,393	$ 452,974
Net income attributable to noncontrolling interest	(54)	(91)	(802)	(170)
Preferred distributions	(1,997)	(1,997)	(3,994)	(3,994)
Net income attributable to Equity Commonwealth common shareholders	25,835	240,289	448,597	448,810
Numerator for earnings per common share - diluted:
Net income	27,886	242,377	453,393	452,974
Net income attributable to noncontrolling interests	(54)	(91)	(802)	(170)
Preferred distributions	(1,997)	0	0	0
Numerator for net income per share - diluted	$ 25,835	$ 242,286	$ 452,591	$ 452,804
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	121,655	122,122	121,901	122,041
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	0	2,563	2,857	2,563
Weighted average number of common shares outstanding — diluted (in shares)	123,255	125,862	126,358	125,841
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.21	$ 1.97	$ 3.68	$ 3.68
Diluted (in dollars per share)	$ 0.21	$ 1.93	$ 3.58	$ 3.60
Anti-dilutive securities:
Preferred shares, dividend percentage	6.50%	6.50%	6.50%	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	150	220	164	203
6.50% Series D Cumulative Convertible Preferred Shares of Beneficial Interest
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	2,857	0	0	0
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	124	32	107	34
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	110	14	92	12
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,524	1,005	1,524	1,064
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	76	172	76	173