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
	Mar. 31, 2016	Mar. 31, 2015
Numerator for earnings per common share - basic and diluted:
Net income	$ 46,402	$ 13,630
Preferred distributions	(6,981)	(6,981)
Net income attributable to Equity Commonwealth common shareholders	$ 39,421	$ 6,649
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	125,840	129,696
Weighted average common shares outstanding — diluted (in shares)	127,522	129,874
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in USD per share)	$ 0.31	$ 0.05
Diluted (in USD per share)	$ 0.31	$ 0.05
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