Document and Entity Information (USD $) In Billions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Feb. 22, 2012	Jun. 30, 2011
Document and Entity Information
Entity Registrant Name	CommonWealth REIT
Entity Central Index Key	0000803649
Document Type	10-K
Document Period End Date	Dec. 31, 2011
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 1.9
Entity Common Stock, Shares Outstanding		83,721,736
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY