CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Rents receivable, allowance for doubtful accounts	$ 7,013	$ 7,885
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)	128,893,552	118,386,918
Common shares of beneficial interest, shares outstanding (in shares)	128,893,552	118,386,918
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)	4,915,497	15,180,000
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,497	15,180,000
Preferred shares, aggregate liquidation preference	122,887	379,500
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