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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Numerator for earnings per common share - basic:
Net (loss) income	$ (5,811)	$ 87,844	$ 18,011	$ 134,246
Net loss (income) attributable to noncontrolling interest	2	0	(6)	0
Preferred distributions	(1,997)	(6,981)	(3,994)	(13,962)
Excess fair value of consideration paid over carrying value of preferred shares	0	(9,609)	0	(9,609)
Net (loss) income attributable to Equity Commonwealth common shareholders	(7,806)	71,254	14,011	110,675
Numerator for earnings per common share - diluted:
Net (loss) income	(5,811)	87,844	18,011	134,246
Preferred distributions	(1,997)	(6,981)	(3,994)	(13,962)
Excess fair value of consideration paid over carrying value of preferred shares	0	(9,609)	0	(9,609)
Numerator for net (loss) income per share - diluted	$ (7,808)	$ 71,254	$ 14,017	$ 110,675
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	124,067	125,508	124,057	125,674
Weighted average number of common shares outstanding — diluted (in shares)	124,067	126,937	125,203	127,229
Net (loss) income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ (0.06)	$ 0.57	$ 0.11	$ 0.88
Diluted (in dollars per share)	$ (0.06)	$ 0.56	$ 0.11	$ 0.87
Series D
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,363	2,363	2,363	2,363
RSUs
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	1,063	0	0	0
LTIP Units
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	126	0	0	0
Series D
Anti-dilutive securities:
Preferred shares, dividend yield	6.50%	6.50%	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)	0	1,429	1,043	1,555
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	0	103	0