CONDENSED CONSOLIDATED BALANCE SHEETS (unaudited) Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Rents receivable, allowance for doubtful accounts	$ 9,281	$ 6,565
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)	126,349,914	129,607,279
Common shares of beneficial interest, shares outstanding (in shares)	126,349,914	129,607,279
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	4,915,497
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,196	4,915,497
Preferred shares, aggregate liquidation preference	$ 122,880	$ 122,887
Series E
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	7.25%	7.25%
Preferred shares of beneficial interest, shares issued (in shares)	11,000,000	11,000,000
Preferred shares, of beneficial interest, shares outstanding (in shares)	11,000,000	11,000,000
Preferred shares, aggregate liquidation preference	$ 275,000	$ 275,000