Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 7,516	$ 6,700
Common Shares of Beneficial Interest - par value, $.03 per share; shares authorized: 275,000; shares issued and outstanding: 128,447 in 2017 and 128,072 in 2016
Common shares of beneficial interest; par value	$ 0.03	$ 0.03
Common shares of beneficial interest, shares authorized (in shares)	275,000,000	275,000,000
Common shares of beneficial interest, shares issued (in shares)	128,447,000	128,072,000
Common shares of beneficial interest, shares outstanding (in shares)	128,447,000	128,072,000