Summary of Significant Accounting Policies (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2011	Dec. 31, 2008	Dec. 31, 2010
Debt Instrument [Line Items]
Unsecured Notes, face amount	$ 650,000,000
Convertible Notes, stated interest rate	6.95%
Summary of Significant Accounting Policies (Textuals) [Abstract]
Deferred Tax Asset	38,700,000
Par value of common shares	$ 0.01				$ 0.01
Partially Owned Consolidated Properties	22
Partially Owned Consolidated Units	4,371
Noncontrolling Interests - Partially Owned Properties	3,295,000				7,991,000
Noncontrolling Interest Settlement Value	63,400,000
Convertible Notes, outstanding	482,500,000
Reduction in Earnings	3,900,000	3,900,000	5,000,000
Amount of Conversion Option	44,300,000
Paid-In Capital Increase				44,300,000
Decrease in Balance of Notes				17,300,000
Decrease in Earnings Per Share	$ 0.01	$ 0.01	$ 0.02
Unamortized cash and conversion option discounts	1,100,000				5,000,000
Carrying amount of conversion option remaining in paid-in capital	44,300,000				44,300,000
Convertible Notes Payable [Member]
Debt Instrument [Line Items]
Unsecured Notes, face amount	650,000,000
Convertible Notes, issuance date	August 2006
Convertible Notes, maturity date	Aug. 01, 2026
Convertible Notes, stated interest rate	3.85%	3.85%
Interest expense recognized in relation to nonconvertible debt	9,300,000	9,300,000
Effective interest rate for conversion option	5.80%
Adjustments for New Accounting Pronouncement [Member] | Restatement Adjustment [Member]
Retained Earnings Adjustments [Line Items]
Retained Earnings Decrease				$ 27,000,000