Selected Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 25,995	$ 26,735	$ 33,368	$ 41,752	$ 42,925	$ 46,873	$ 48,636	$ 58,588	$ 127,850	$ 197,022	$ 340,571
Net income attributable to Equity Commonwealth common shareholders	$ 13,993	$ 21,889	$ 240,289	$ 208,521	$ 13,420	$ 30,767	$ 35,036	$ 185,602	$ 484,692	$ 264,825	$ 21,668
Net income attributable to Equity Commonwealth common shareholders per share—basic (in dollars per share)	$ 0.11	$ 0.18	$ 1.97	$ 1.71	$ 0.11	$ 0.25	$ 0.29	$ 1.50
Net income attributable to Equity Commonwealth common shareholders per share—diluted (in dollars per share)	$ 0.11	$ 0.18	$ 1.93	$ 1.67	$ 0.11	$ 0.25	$ 0.29	$ 1.48