Equity and Accumulated Other Comprehensive Loss, Net (Narrative) (Details) (USD $)	6 Months Ended		12 Months Ended	1 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Equity Distribution Agreement [Member]	Jun. 30, 2011 Equity Distribution Agreement [Member]	Jan. 22, 2010 Equity Distribution Agreement [Member]
Aggregate offering price of the distribution agreement maximum				$ 400,000,000	$ 400,000,000	$ 54,300,000
Aggregate shares issued from the distribution agreement				3,000,000	3,000,000	1,100,000
Common stock sold	98,754,727		91,159,221	6,800,000	6,800,000
Gross proceeds from the sale of common stock	179,583,000	461,868,000		400,000,000
Net proceeds from sale of common stock				394,000,000	176,900,000
Price per share sold as part of the distribution agreement					$ 60.51
Payment of commissions to sales agents					2,700,000
Redemption value of OP units	272,000,000		244,500,000
Available indebtedness	17,800,000
Acquisition of noncontrolling ownership in joint venture	$ 53,200,000