Shareholders' Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Reconciliation of changes in our shareholders equity
Balance at the beginning of the period			$ 3,568,517
Net income	20,586	19,964	45,160	66,576
Other comprehensive income:
Unrealized loss on derivative instrument	(2,404)	(4,119)	(2,441)	(2,074)
Foreign currency translation adjustments	(3,447)	11,343	1,081	15,705
Decrease in share of investees other comprehensive income	(3)	40	(4)	44
Total comprehensive income	14,732	27,228	43,796	80,233
Issuance of shares of subsidiary, net			180,814
Share grants			187
Distributions			(111,188)
Balance at the end of the period	3,682,126		3,682,126
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			39,745
Other comprehensive income:
Unrealized loss on derivative instrument			(2,441)
Foreign currency translation adjustments			1,081
Decrease in share of investees other comprehensive income			(4)
Total comprehensive income			38,381
Issuance of shares of subsidiary, net			(23,856)
Share grants			187
Distributions			(111,188)
Balance at the end of the period	3,472,041		3,472,041
Shareholders' Equity Attributable to Noncontrolling Interest
Reconciliation of changes in our shareholders equity
Net income			5,415
Other comprehensive income:
Total comprehensive income			5,415
Issuance of shares of subsidiary, net			204,670
Balance at the end of the period	$ 210,085		$ 210,085