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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator for earnings per common share - basic and diluted:
Income from continuing operations									$ 232,894	$ 99,857	$ 21,206
Preferred distributions									(17,956)	(27,924)	(32,095)
Excess fair value of consideration paid over carrying value of preferred shares									(9,609)	0	0
Excess fair value of consideration over carrying value of preferred shares									0	0	(16,205)
Income (loss) from continuing operations attributable to Equity Commonwealth common shareholders									205,329	71,933	(27,094)
Discontinued operations									0	0	2,806
Net income (loss) attributable to Equity Commonwealth common shareholders	$ 10,263	$ 84,391	$ 71,254	$ 39,421	$ 36,164	$ 23,485	$ 5,635	$ 6,649	$ 205,329	$ 71,933	$ (24,288)
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)									125,474	128,621	125,163
Weighted average common shares outstanding — diluted (in shares)									126,768	129,437	125,163
Earnings per share - basic:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)									$ 1.64	$ 0.56	$ (0.21)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Net income (loss) (in dollars per share)	$ 0.08	$ 0.67	$ 0.57	$ 0.31	$ 0.29	$ 0.18	$ 0.04	$ 0.05	1.64	0.56	(0.19)
Earnings per share - diluted:
Income from continuing operations attributable to Equity Commonwealth common shareholders (in dollars per share)									1.62	0.56	(0.21)
Discontinued operations (in dollars per share)									0.00	0.00	0.02
Net income (loss) (in dollars per share)	$ 0.08	$ 0.67	$ 0.56	$ 0.31	$ 0.28	$ 0.18	$ 0.04	$ 0.05	$ 1.62	$ 0.56	$ (0.19)
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	2,363	4,175
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%