Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accounts receivable, allowance for doubtful accounts	$ 1,882	$ 1,861
Preferred shares, par (in usd per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized	2,000,000	2,000,000
Common shares of beneficial interest, par (in usd per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized	120,000,000	120,000,000
Common shares of beneficial interest, shares issued	79,366,000	79,272,000
Common shares of beneficial interest, shares outstanding	79,366,000	79,272,000
Series D Preferred Shares
Cumulative convertible perpetual preferred shares, shares issued	1,849,000	1,849,000
Cumulative convertible perpetual preferred shares, shares outstanding	1,849,000	1,849,000
Cumulative convertible perpetual preferred shares, liquidation preference (in usd per share)	$ 50	$ 50
Cumulative convertible perpetual preferred shares, dividend rate (as a percent)	7.25%	7.25%