Noncontrolling Interests (Outstanding Cumulative Redeemable Preferred Units) (Details) (USD $)			9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Series B Preferred Stock [Member] percentage	Sep. 30, 2011 Series E Preferred Stock [Member] percentage	Sep. 30, 2011 Series F Preferred Stock [Member] percentage	Sep. 30, 2011 Series G Preferred Stock [Member] percentage	Sep. 30, 2011 Series H Preferred Stock [Member] percentage
Noncontrolling Interest in Operating Partnerships			$ 95,000,000	$ 20,000,000	$ 50,000,000	$ 27,000,000	$ 100,000,000
Redeemable Preferred Stock, Amount	3,808,746	3,928,733
Redeemable Preferred Stock, Units Outstanding	9,740,000	9,740,000	3,800,000	400,000	1,000,000	540,000	4,000,000
Preferred Stock, Liquidation Preference			$ 25	$ 50	$ 50	$ 50	$ 25
Preferred Stock, Dividend Rate			7.45%	7.00%	6.65%	6.70%	7.40%
Preferred Stock Redeemable As Of			Aug. 31, 2009	Jun. 16, 2010	Jun. 30, 2010	Dec. 15, 2011	Aug. 21, 2012
Preferred Stock, Exchangeable After			8/31/13 into Series B Cumulative Redeemable Preferred Shares of the Trust	6/16/15 into Series E Cumulative Redeemable Preferred Shares of the Trust	12/12/15 into Series F Cumulative Redeemable Preferred Shares of the Trust	12/15/16 into Series G Cumulative Redeemable Preferred Shares of the Trust	8/21/17 into Series H Cumulative Redeemable Preferred Shares of the Trust