CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Preferred shares, par (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized	2,000,000	2,000,000
Common shares of beneficial interest, par (in dollars 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	79,272,000	79,162,000
Common shares of beneficial interest, shares outstanding	79,272,000	79,162,000
Series D Preferred Stock
Preferred shares, par (in dollars per share)	$ 0.01
Cumulative convertible perpetual preferred shares, liquidation preference (in dollars per share)	$ 50	$ 50
Cumulative convertible perpetual preferred shares, shares issued	1,848,539	1,849,000
Cumulative convertible perpetual preferred shares, shares outstanding	1,849,000	1,849,000
Cumulative convertible perpetual preferred shares, dividend rate percentage	7.25%	7.25%