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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator for earnings per common share - basic:
Net income									$ 272,908	$ 29,666	$ 232,894
Net income attributable to noncontrolling interest									(95)	(10)	0
Preferred distributions									(7,988)	(7,988)	(17,956)
Excess fair value of consideration paid over carrying value of preferred shares									0	0	(9,609)
Net income attributable to Equity Commonwealth common shareholders	$ 13,420	$ 30,767	$ 35,036	$ 185,602	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	264,825	21,668	205,329
Net Income (Loss) Available to Common Stockholders, Diluted [Abstract]
Net income									272,908	29,666	232,894
Net income attributable to noncontrolling interest									(95)	0	0
Preferred distributions									(7,988)	(7,988)	(17,956)
Excess fair value of consideration paid over carrying value of preferred shares									0	0	(9,609)
Numerator for net income per share - diluted									$ 264,825	$ 21,678	$ 205,329
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									122,314	124,125	125,474
Weighted average common shares outstanding — diluted (in shares)									123,385	125,129	126,768
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)									$ 2.17	$ 0.17	$ 1.64
Diluted (in dollars per share)									$ 2.15	$ 0.17	$ 1.62
Series D
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									2,563	2,363	2,363
Preferred shares, dividend yield									6.50%	6.50%	6.50%
LTIP units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									43	0	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									1	0	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									956	912	1,294
LTIP units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									115	92	0