Shareholders' Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Reconciliation of changes in our shareholders equity
Balance at the beginning of the period	$ 3,568,517
Other comprehensive income:
Net income	24,574	46,612
Unrealized loss on derivative instrument	(37)	2,045
Foreign currency translation adjustments	4,528
Decrease in share of investees other comprehensive income	(1)
Total comprehensive income	29,064	53,005
Issuance of shares of subsidiary, net	180,954
Distributions	(55,505)
Balance at the end of the period	3,723,030
Shareholders' Equity Attributable to CommonWealth REIT
Reconciliation of changes in our shareholders equity
Balance at the beginning of the period	3,568,517
Other comprehensive income:
Net income	23,680
Unrealized loss on derivative instrument	(37)
Foreign currency translation adjustments	4,528
Decrease in share of investees other comprehensive income	(1)
Total comprehensive income	28,170
Issuance of shares of subsidiary, net	(23,758)
Distributions	(55,505)
Balance at the end of the period	3,517,424
Shareholders' Equity Attributable to Noncontrolling Interest
Other comprehensive income:
Net income	894
Total comprehensive income	894
Issuance of shares of subsidiary, net	204,712
Balance at the end of the period	$ 205,606