Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 6,944	$ 7,516
Common Shares of Beneficial Interest - par value, $.03 per share; shares authorized: 275,000; shares issued and outstanding: 128,074 in 2018 and 128,447 in 2017
Common Shares of Beneficial Interest; par value (dollars per share)	$ 0.03	$ 0.03
Common Shares of Beneficial Interest - shares authorized (shares)	275,000,000	275,000,000
Common Shares of Beneficial Interest - shares issued (shares)	128,074,000	128,447,000
Common Shares of Beneficial Interest - shares outstanding (shares)	128,074,000	128,447,000