Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Numerator for earnings per common share - basic:
Net income (loss)	$ 37,357	$ (16,429)	$ 452,093
Net (income) loss attributable to noncontrolling interest	(94)	33	(799)
Preferred distributions	(7,988)	(7,988)	(7,988)
Net income (loss) attributable to Equity Commonwealth common shareholders	29,275	(24,384)	443,306
Numerator for earnings per common share - diluted:
Net income (loss)	37,357	(16,429)	452,093
Net (income) loss attributable to noncontrolling interest	(94)	33	(799)
Preferred distributions	(7,988)	(7,988)	0
Numerator for net income (loss) per share - diluted	$ 29,275	$ (24,384)	$ 451,294
Denominator for earnings per common share - basic and diluted:
Weighted average common shares outstanding — basic (in shares)	111,674	121,411	121,786
Weighted average common shares outstanding — diluted (in shares)	112,825	121,411	126,606
Net income (loss) per common share attributable to Equity Commonwealth common shareholders, basic
Basic (in dollars per share)	$ 0.26	$ (0.20)	$ 3.64
Net income (loss) per common share attributable to Equity Commonwealth common shareholders, diluted
Diluted (in dollars per share)	$ 0.26	$ (0.20)	$ 3.56
Anti-dilutive securities(4):
Preferred shares, dividend yield	6.50%	6.50%
Weighted average number of shares, restricted stock units, unvested (in shares)	105	256	157
Series D Preferred Stock
Denominator for earnings per common share - basic and diluted:
Series D preferred shares; 6.50% cumulative convertible (in shares)	0	0	3,237
Anti-dilutive securities(4):
Preferred shares, dividend yield	6.50%	6.50%	6.50%
Series D Preferred Stock
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	3,365	3,237	0
RSUs
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	0	549	0
LTIP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	0	84	119
OP Units
Anti-dilutive securities(4):
Effect of anti-dilutive securities (in shares)	276	208	102
RSUs
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	970	0	1,508
LTIP Units
Denominator for earnings per common share - basic and diluted:
Weighted average number of common shares outstanding, dilutive adjustment (in shares)	181	0	75