Shareholders' Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reconciliation of changes in our shareholders equity
Balances			$ 3,501,468
Net (loss) income	(216,207)	22,265	(151,666)	67,425
Other comprehensive income (loss):
Unrealized gain on derivative instruments	178	454	4,011	(1,987)
Foreign currency translation adjustments	5,455	4,516	(31,393)	5,597
Equity in unrealized loss of an investee	13	69	(149)	65
Total comprehensive (loss) income	(210,561)	27,304	(179,197)	71,100
Issuance of common shares, net			(626,809)	(180,814)
Share grants			1,819
Distributions			(128,422)
Deconsolidation of a subsidiary			(401,444)
Balances	3,421,033		3,421,033
Shareholders' Equity Attributable to CommonWealth REIT
Reconciliation of changes in our shareholders equity
Balances			3,105,428
Net (loss) income			(171,759)
Other comprehensive income (loss):
Unrealized gain on derivative instruments			4,011
Foreign currency translation adjustments			(31,393)
Equity in unrealized loss of an investee			(113)
Total comprehensive (loss) income			(199,254)
Issuance of common shares, net			626,851
Share grants			1,567
Distributions			(113,559)
Balances	3,421,033		3,421,033
Shareholders' Equity Attributable to Noncontrolling Interest
Reconciliation of changes in our shareholders equity
Balances			396,040
Net (loss) income			20,093
Other comprehensive income (loss):
Equity in unrealized loss of an investee			(36)
Total comprehensive (loss) income			20,057
Issuance of common shares, net			(42)
Share grants			252
Distributions			(14,863)
Deconsolidation of a subsidiary			$ (401,444)