Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 153	$ 245
Preferred shares of beneficial interest, liquidation preference value	$ 250,000	$ 250,000
Preferred shares of beneficial interest, par value (usd per share)	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized	100,000,000	100,000,000
Preferred shares of beneficial interest, shares issued	10,000,000	10,000,000
Preferred shares of beneficial interest, shares outstanding	10,000,000	10,000,000
Common shares of beneficial interest, par value (usd per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized	500,000,000	500,000,000
Common shares of beneficial interest, shares issued	68,912,185	68,812,575
Common shares of beneficial interest, shares outstanding	68,912,185	68,812,575