Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 6,700	$ 6,072
Common Shares of Beneficial Interest - par value, $.03 per share; shares authorized: 275,000; shares issued and outstanding: 123,951 in 2015 and 122,489 in 2014
Common shares of beneficial interest, par value (in dollars per share)	$ 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)	128,072,000	123,951,000
Common shares of beneficial interest, shares outstanding (in shares)	128,072,000	123,951,000