Common Shares Of Beneficial Interest (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	May 31, 2010
Common Shares Of Beneficial Interest
Common dividend per share	$ 0.275	$ 0.26	$ 0.55	$ 0.52
Common shares of beneficial interest - par value	$ 0.03		$ 0.03		$ 0.03	$ 0.03
Common shares authorized prior to amendment						150,000,000
Common shares of beneficial interest - shares authorized	275,000,000		275,000,000		275,000,000	275,000,000