CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Rents receivable, allowance for doubtful accounts	$ 6,565,000	$ 7,885,000
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)	129,607,279	118,386,918
Common shares of beneficial interest, shares outstanding (in shares)	129,607,279	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	122,887,000	379,500,000
Common shares of beneficial interest, shares issued (in shares)	4,915,497	15,180,000
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	$ 275,000,000	$ 275,000,000
Common shares of beneficial interest, shares issued (in shares)	11,000,000	11,000,000
Unvested restricted shares
Common shares of beneficial interest, shares outstanding (in shares)	710,182	130,914