CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 32,777	$ 33,224	$ 257,486	$ 51,235
Other comprehensive income (loss), net of tax:
Unrealized (loss) gain on derivative instruments	0	(19)	456	(280)
Unrealized gain on marketable securities	463	1,455	535	3,159
Total comprehensive income	33,240	34,660	258,477	54,114
Comprehensive income attributable to the noncontrolling interest	(13)	(12)	(90)	(18)
Total comprehensive income attributable to Equity Commonwealth	$ 33,227	$ 34,648	$ 258,387	$ 54,096