CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 33,224	$ 86,388	$ 51,235	$ 220,634
Other comprehensive (loss) income, net of tax:
Unrealized (loss) gain on derivative instruments	(19)	1,117	(280)	2,570
Unrealized gain on marketable securities	1,455	0	3,159	0
Total comprehensive income	34,660	87,505	54,114	223,204
Comprehensive income attributable to the noncontrolling interest	(12)	0	(18)	0
Total comprehensive income attributable to Equity Commonwealth	$ 34,648	$ 87,505	$ 54,096	$ 223,204