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		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Numerator for earnings per common share - basic and diluted:
Net income	$ 86,388	$ 30,466	$ 220,634	$ 56,712
Preferred distributions	(1,997)	(6,981)	(15,959)	(20,943)
Excess fair value of consideration paid over carrying value of preferred shares	0	0	(9,609)	0
Net income attributable to Equity Commonwealth common shareholders	$ 84,391	$ 23,485	$ 195,066	$ 35,769
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	125,533	128,739	125,627	129,386
Weighted average number of common shares outstanding — diluted (in shares)	126,568	129,878	127,009	130,093
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in USD per share)	$ 0.67	$ 0.18	$ 1.55	$ 0.28
Diluted (in USD per share)	$ 0.67	$ 0.18	$ 1.54	$ 0.27
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%