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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator for earnings per common share - basic and diluted:
Income (loss) from continuing operations									$ 99,857	$ 21,206	$ (37,318)
Net income attributable to noncontrolling interest in consolidated subsidiary									0	0	(20,093)
Preferred distributions									(27,924)	(32,095)	(44,604)
Excess fair value of consideration over carrying value of preferred shares									0	(16,205)	0
Income (loss) from continuing operations attributable to Equity Commonwealth common shareholders									71,933	(27,094)	(102,015)
Discontinued operations									0	2,806	(119,649)
Numerator for net income (loss) per share - basic and diluted	$ 36,164	$ 23,485	$ 5,635	$ 6,649	$ (165,542)	$ 149,759	$ (17,802)	$ 9,297	$ 71,933	$ (24,288)	$ (221,664)
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									128,621	125,163	112,378
Weighted average common shares outstanding — diluted (in shares)									129,437	125,163	112,378
Earnings per share - basic:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)									$ 0.56	$ (0.21)	$ (0.91)
Discontinued operations (in dollars per share)									0.00	0.02	(1.06)
Net income (loss) (in dollars per share)	$ 0.29	$ 0.18	$ 0.04	$ 0.05	$ (1.28)	$ 1.16	$ (0.14)	$ 0.08	0.56	(0.19)	(1.97)
Earnings per share - diluted:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)									0.56	(0.21)	(0.91)
Discontinued operations (in dollars per share)									0.00	0.02	(1.06)
Net income (loss) (in dollars per share)	$ 0.28	$ 0.18	$ 0.04	$ 0.05	$ (1.28)	$ 1.16	$ (0.14)	$ 0.08	$ 0.56	$ (0.19)	$ (1.97)
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Preferred shares, dividend yield									6.50%	6.50%	6.50%
Anti-dilutive securities:
Anti-dilutive securities (in shares)									2,363	4,175	7,298
Effect of Series D preferred shares; 6 1/2% cumulative convertible
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Preferred shares, dividend yield									6.50%	6.50%