Shareholders Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended			9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Series J Preferred Stock [Member]	Apr. 20, 2011 Series J Preferred Stock [Member]	Aug. 05, 2011 Series J Preferred Stock [Member] Public Offering [Member]	May 05, 2011 Series J Preferred Stock [Member] Public Offering [Member]	Apr. 20, 2011 Series J Preferred Stock [Member] Public Offering [Member]	Apr. 20, 2011 Series J Preferred Stock [Member] Over Allotment [Member]
Class Of Stock [Line Items]
Preferred shares of beneficial interest: outstanding shares	42,186,709		32,340,009
Preferred Stock Par Or Stated Value Per Share					$ 25.00
Preferred Stock Dividend Rate Percentage Or Per Dollar Amount				6.875%
Preferred shares of beneficial interest: authorized shares	110,000,000		110,000,000
Preferred shares of beneficial interest: issued shares	42,186,709		32,340,009			1,000,000	800,000	7,000,000	1,050,000
Preferred Stock, Value, Issued	$ 1,021,855		$ 783,088
Proceeds From Issuance Of Preferred Stock And Preference Stock	$ 239,037	$ 0
Preferred Stock Redemption Price Per Share					$ 25.00	$ 25.00	$ 25.00
Preferred Stock Issued Additional Disclosure				We retained aggregate net proceeds of $239,037,000, after underwriters’ discounts and issuance costs and contributed the net proceeds to the Operating Partnership in exchange for 9,850,000 Series J Preferred Units (with economic terms that mirror those of the Series J Preferred Shares)
Preferred Stock, Redemption Terms				The Series J Preferred Shares are not convertible into, or exchangeable for, any of our properties or securities. On or after five years from the date of issuance (or sooner under limited circumstances), we, at our option, may redeem the Series J Preferred Shares at a redemption price of $25.00 per share, plus accrued and unpaid dividends through the date of redemption.
Preferred Units, Issued				9,850,000