Shareholders' Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Reconciliation of changes in our shareholders equity
Balance at the beginning of the period			$ 3,568,517
Net income	22,265	28,535	67,425	95,111
Other comprehensive income:
Unrealized loss on derivative instrument	454	(6,577)	(1,987)	(8,651)
Foreign currency translation adjustments	4,516	(33,289)	5,597	(17,584)
Increase in share of investees other comprehensive income	69	14	65	58
Total comprehensive income (loss)	27,304	(11,317)	71,100	68,916
Issuance of shares of subsidiary, net			180,814
Redemption of preferred shares			(150,000)
Share grants			1,347
Distributions			(171,654)
Balance at the end of the period	3,500,124		3,500,124
Shareholders' Equity Attributable to CommonWealth REIT
Reconciliation of changes in our shareholders equity
Balance at the beginning of the period			3,568,517
Net income			57,363
Other comprehensive income:
Unrealized loss on derivative instrument			(1,987)
Foreign currency translation adjustments			5,597
Increase in share of investees other comprehensive income			55
Total comprehensive income (loss)			61,028
Issuance of shares of subsidiary, net			(24,369)
Redemption of preferred shares			(150,000)
Share grants			1,240
Distributions			(167,146)
Balance at the end of the period	3,289,270		3,289,270
Shareholders' Equity Attributable to Noncontrolling Interest
Reconciliation of changes in our shareholders equity
Net income			10,062
Other comprehensive income:
Increase in share of investees other comprehensive income			10
Issuance of shares of subsidiary, net			205,183
Share grants			107
Distributions			(4,508)
Balance at the end of the period	$ 210,854		$ 210,854