CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 29,666	$ 232,894	$ 99,857
Other comprehensive income (loss), net of tax:
Unrealized (loss) gain on derivative instruments and other assets	(248)	3,479	612
Foreign currency translation adjustments	0	0	48,917
Unrealized gain, net on marketable securities	361	0	0
Total comprehensive income	29,779	236,373	149,386
Comprehensive income attributable to the noncontrolling interest	(10)	0	0
Total comprehensive income attributable to Equity Commonwealth	$ 29,769	$ 236,373	$ 149,386