Shareholders' Equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reconciliation of changes in our shareholders equity
Balance	$ 3,501,468
Net income	35,624	24,574
Other comprehensive income (loss):
Unrealized gain on derivative instruments	1,051	(37)
Foreign currency translation adjustments	973	4,528
Equity in unrealized loss of an investee	(16)	(1)
Total comprehensive income	37,632	29,064
Issuance of common shares, net	(626,991)	(180,954)
Distributions	(39,361)
Balance	4,126,730
Shareholders' Equity Attributable to CommonWealth REIT
Reconciliation of changes in our shareholders equity
Balance	3,105,428
Net income	25,667
Other comprehensive income (loss):
Unrealized gain on derivative instruments	1,051
Foreign currency translation adjustments	973
Equity in unrealized loss of an investee	(12)
Total comprehensive income	27,679
Issuance of common shares, net	627,027
Distributions	(32,102)
Balance	3,728,032
Shareholders' Equity Attributable to Noncontrolling Interest
Reconciliation of changes in our shareholders equity
Balance	396,040
Net income	9,957
Other comprehensive income (loss):
Equity in unrealized loss of an investee	(4)
Total comprehensive income	9,953
Issuance of common shares, net	(36)
Distributions	(7,259)
Balance	$ 398,698