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
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for earnings per common share - basic:
Net income									$ 29,666	$ 232,894	$ 99,857
Net income attributable to noncontrolling interest									(10)	0	0
Preferred distributions									(7,988)	(17,956)	(27,924)
Excess fair value of consideration paid over carrying value of preferred shares									0	(9,609)	0
Net income attributable to Equity Commonwealth common shareholders	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	$ 10,263	$ 84,391	$ 71,254	$ 39,421	21,668	205,329	71,933
Net Income (Loss) Available to Common Stockholders, Diluted [Abstract]
Net income									29,666	232,894	99,857
Preferred distributions									(7,988)	(17,956)	(27,924)
Excess fair value of consideration paid over carrying value of preferred shares									0	(9,609)	0
Numerator for net income per share - diluted									$ 21,678	$ 205,329	$ 71,933
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									124,125	125,474	128,621
Weighted average common shares outstanding — diluted (in shares)									125,129	126,768	129,437
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)									$ 0.17	$ 1.64	$ 0.56
Diluted (in dollars per share)									$ 0.17	$ 1.62	$ 0.56
Series D
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)									2,363	2,363	2,363
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)									912	1,294	816
LTIP units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									92	0	0