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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 71,623	$ 77,798	$ 91,599	$ 99,551	$ 103,546	$ 114,632	$ 145,367	$ 137,135	$ 340,571	$ 500,680	$ 714,891
Net income (loss) attributable to Equity Commonwealth common shareholders	$ (23,558)	$ 31,215	$ (7,806)	$ 21,817	$ 10,263	$ 84,391	$ 71,254	$ 39,421	$ 21,668	$ 205,329	$ 71,933
Net income (loss) attributable to Equity Commonwealth common shareholders per share—basic (in dollars per share)	$ (0.19)	$ 0.25	$ (0.06)	$ 0.18	$ 0.08	$ 0.67	$ 0.57	$ 0.31
Net income (loss) attributable to Equity Commonwealth common shareholders per share—diluted (in dollars per share)	$ (0.19)	$ 0.25	$ (0.06)	$ 0.17	$ 0.08	$ 0.67	$ 0.56	$ 0.31