CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Rents receivable, allowance for doubtful accounts	$ 4,771	$ 5,105
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized (in shares)	350,000,000	350,000,000
Common shares of beneficial interest, shares issued (in shares)	124,217,616	123,994,465
Common shares of beneficial interest, shares outstanding (in shares)	124,217,616	123,994,465
Series D
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized (in shares)	50,000,000	50,000,000
Preferred shares, dividend yield	6.50%	6.50%
Preferred shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,196	4,915,196
Preferred shares, aggregate liquidation preference	$ 122,880	$ 122,880
Common shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196