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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator for earnings per common share - basic:
Net income									$ 492,866	$ 272,908	$ 29,666
Net income attributable to noncontrolling interest									(186)	(95)	(10)
Preferred distributions									(7,988)	(7,988)	(7,988)
Net income attributable to Equity Commonwealth common shareholders	$ 13,993	$ 21,889	$ 240,289	$ 208,521	$ 13,420	$ 30,767	$ 35,036	$ 185,602	484,692	264,825	21,668
Numerator for earnings per common share - diluted:
Net income									492,866	272,908	29,666
Net income attributable to noncontrolling interest									(186)	(95)	0
Preferred distributions									0	(7,988)	(7,988)
Numerator for net income per share - diluted									$ 492,680	$ 264,825	$ 21,678
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									122,091	122,314	124,125
Weighted average common shares outstanding — diluted (in shares)									126,260	123,385	125,129
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)									$ 3.97	$ 2.17	$ 0.17
Diluted (in dollars per share)									$ 3.90	$ 2.15	$ 0.17
Series D Preferred Stock
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6.50% cumulative convertible (in shares)									2,857	0	0
Series D Preferred Stock
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									0	2,563	2,363
Preferred shares, dividend yield									6.50%	6.50%	6.50%
LTIP units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									33	43	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)									14	1	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									1,138	956	912
LTIP units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)									174	115	92