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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Numerator for earnings per common share - basic and diluted:
Net income	$ 87,844	$ 12,616	$ 134,246	$ 26,246
Preferred distributions	(6,981)	(6,981)	(13,962)	(13,962)
Excess fair value of consideration paid over carrying value of preferred shares	(9,609)	0	(9,609)	0
Net income attributable to Equity Commonwealth common shareholders	$ 71,254	$ 5,635	$ 110,675	$ 12,284
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	125,508	129,733	125,674	129,714
Weighted average number of common shares outstanding — diluted (in shares)	126,937	130,537	127,229	130,205
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in USD per share)	$ 0.57	$ 0.04	$ 0.88	$ 0.09
Diluted (in USD per share)	$ 0.56	$ 0.04	$ 0.87	$ 0.09
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,363	2,363	2,363	2,363
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Preferred shares, dividend yield	6.50%	6.50%	6.50%	6.50%	6.50%