Common Shares Of Beneficial Interest (Details) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Common dividend per share	$ 0.290	$ 0.275	$ 1.16	$ 1.10	$ 1.04
Future common dividend per share	$ 0.305