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, 2019	Mar. 31, 2018	Dec. 31, 2018
Numerator for earnings per common share - basic:
Net income	$ 210,597	$ 187,662
Net income attributable to noncontrolling interest	(79)	(63)
Preferred distributions	(1,997)	(1,997)
Net income attributable to Equity Commonwealth common shareholders	208,521	185,602
Numerator for earnings per common share - diluted:
Net income	210,597	187,662
Net income attributable to noncontrolling interests	(79)	0
Numerator for net income per share - diluted	$ 210,518	$ 187,662
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding — basic (in shares)	121,960	123,867
Weighted average number of common shares outstanding — diluted (in shares)	125,822	127,097
Net income per common share attributable to Equity Commonwealth common shareholders:
Basic (in dollars per share)	$ 1.71	$ 1.50
Diluted (in dollars per share)	$ 1.67	$ 1.48
LTIP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	42	0
OP Units
Anti-dilutive securities:
Effect of anti-dilutive securities (in shares)	4	0
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	1,124	730
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	175	136
OP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	0	1
Series D
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6 1/2% cumulative convertible (in shares)	2,563	2,363
Anti-dilutive securities:
Preferred shares, dividend yield	6.50%	6.50%	6.50%