Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Rents receivable, allowance for doubtful accounts (in dollars)	$ 12,575	$ 12,550
Preferred shares of beneficial interest, shares authorized	50,000,000	50,000,000
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
Series C
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, dividend yield (as a percent)	7.125%	7.125%
Preferred shares of beneficial interest, shares issued	6,000,000	6,000,000
Preferred shares of beneficial interest, shares outstanding	6,000,000	6,000,000
Preferred shares, aggregate liquidation preference (in dollars)	150,000	150,000
Series D
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, dividend yield (as a percent)	6.50%	6.50%
Preferred shares of beneficial interest, shares issued	15,180,000	15,180,000
Preferred shares of beneficial interest, shares outstanding	15,180,000	15,180,000
Preferred shares, aggregate liquidation preference (in dollars)	379,500	379,500
Series E
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, dividend yield (as a percent)	7.25%
Preferred shares of beneficial interest, shares issued	11,000,000	0
Preferred shares of beneficial interest, shares outstanding	11,000,000	0
Preferred shares, aggregate liquidation preference (in dollars)	$ 275,000	$ 0