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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator for earnings per common share - basic:
Net income (loss)	$ 37,047	$ (5,811)	$ 224,709	$ 18,011
Net (income) loss attributable to noncontrolling interest	(14)	2	(77)	(6)
Preferred distributions	(1,997)	(1,997)	(3,994)	(3,994)
Net income (loss) attributable to Equity Commonwealth common shareholders	35,036	(7,806)	220,638	14,011
Numerator for earnings per common share - diluted:
Net income (loss)	37,047	(5,811)	224,709	18,011
Net income attributable to noncontrolling interests	(14)	0	(77)	0
Preferred distributions	(1,997)	(1,997)	0	(3,994)
Numerator for net income (loss) per share - diluted	$ 35,036	$ (7,808)	$ 224,632	$ 14,017
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	121,822	124,067	122,839	124,057
Weighted average number of common shares outstanding — diluted (in shares)	122,649	124,067	126,027	125,203
Net income (loss) per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 0.29	$ (0.06)	$ 1.80	$ 0.11
Diluted (in dollars per share)	$ 0.29	$ (0.06)	$ 1.78	$ 0.11
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities	2,363	2,363	0	2,363
Preferred shares, dividend yield			6.50%	6.50%
RSUs
Anti-dilutive securities:
Effect of anti-dilutive securities	0	1,063	0	0
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities	42	126	42	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities	1	0	1	0
Series D
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	0	0	2,363	0
Anti-dilutive securities:
Preferred shares, dividend yield			6.50%	6.50%	6.50%
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	762	0	746	1,043
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	65	0	79	103