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CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Statement of Financial Position [Abstract]    
Common shares of beneficial interest, par value (in dollars per share) $ 0.01 $ 0.01
Common shares of beneficial interest, shares authorized (in shares) 300,000,000 300,000,000
Common shares of beneficial interest, shares issued (in shares) 237,730,900 237,630,409
Common shares of beneficial interest, shares outstanding (in shares) 237,730,900 237,630,409