CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Series C	Dec. 31, 2010 Series C	Sep. 30, 2011 Series D	Dec. 31, 2010 Series D	Sep. 30, 2011 Series E	Dec. 31, 2010 Series E
Rents receivable, allowance for doubtful accounts (in dollars)	$ 12,421	$ 12,550
Preferred shares of beneficial interest, par value (in dollars per share)			$ 0.01	$ 0.01			$ 0.01
Preferred shares of beneficial interest, par value (in dollars per share)					$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized	50,000,000	50,000,000
Preferred shares, dividend yield (as a percent)			7.125%	7.125%	6.50%	6.50%	7.25%
Preferred shares of beneficial interest, shares issued			6,000,000	6,000,000	15,180,000	15,180,000	11,000,000	0
Preferred shares of beneficial interest, shares outstanding			6,000,000	6,000,000	15,180,000	15,180,000	11,000,000	0
Preferred shares, aggregate liquidation preference (in dollars)			$ 150,000	$ 150,000	$ 379,500	$ 379,500	$ 275,000	$ 0
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized	350,000,000	350,000,000
Common shares of beneficial interest, shares issued	83,721,736	72,138,686
Common shares of beneficial interest, shares outstanding	83,721,736	72,138,686