CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2013	Dec. 31, 2012
CONDENSED CONSOLIDATED BALANCE SHEETS
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized	70,000,000	70,000,000
Common shares of beneficial interest, shares issued	54,664,118	54,643,888
Common shares of beneficial interest, shares outstanding	54,664,118	54,643,888