Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Cumulative Preferred Shares Of Beneficial Interest
Par value	$ 0.01	$ 0.01
Shares authorized	100,000,000	100,000,000
Shares issued (in series)	142,500	113,500
Shares outstanding	142,500	113,500
Common Shares Of Beneficial Interest
Par value	$ 0.10	$ 0.10
Shares authorized	650,000,000	650,000,000
Shares issued	171,776,291	171,388,286
Shares outstanding	171,776,291	171,388,286