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, 2018	Mar. 31, 2017	Dec. 31, 2017
Numerator for earnings per common share - basic:
Net income	$ 187,662	$ 23,822
Net income attributable to noncontrolling interest	(63)	(8)
Preferred distributions	(1,997)	(1,997)
Net income attributable to Equity Commonwealth common shareholders	185,602	21,817
Numerator for earnings per common share - diluted:
Net income	187,662	23,822
Preferred distributions	0	(1,997)
Numerator for net income per share - diluted	$ 187,662	$ 21,825
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	123,867	124,047
Weighted average number of common shares outstanding — diluted (in shares)	127,097	125,150
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 1.50	$ 0.18
Diluted (in dollars per share)	$ 1.48	$ 0.17
Series D
Anti-dilutive securities:
Effect of Series D preferred shares; 6 1/2% cumulative convertible (in shares)	0	2,363
Preferred shares, dividend yield	6.50%	6.50%
Series D
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible	2,363	0
Anti-dilutive securities:
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)	730	1,023
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	136	80
OP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1	0