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		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Numerator for earnings per common share - basic:
Net income	$ 23,822	$ 46,402
Net income attributable to noncontrolling interest	(8)	0
Preferred distributions	(1,997)	(6,981)
Net income attributable to Equity Commonwealth common shareholders	21,817	39,421
Numerator for earnings per common share - diluted:
Net income	23,822	46,402
Preferred distributions	(1,997)	(6,981)
Net Income (Loss) Available to Common Stockholders, Diluted	$ 21,825	$ 39,421
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	124,047	125,840
Weighted average number of common shares outstanding — diluted (in shares)	125,150	127,522
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in USD per share)	$ 0.18	$ 0.31
Diluted (in USD per share)	$ 0.17	$ 0.31
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
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%
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,023	1,682
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	80	0