Selected Quarterly Financial Data - Summary of Unaudited Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 103,546	$ 114,632	$ 145,367	$ 137,135	$ 138,934	$ 159,208	$ 203,694	$ 213,055	$ 500,680	$ 714,891	$ 861,857
Net income 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)
Net income attributable to Equity Commonwealth common shareholders per share—basic (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)
Net income attributable to Equity Commonwealth common shareholders per share—diluted (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)