Debt Mortgage Notes Payable (Details) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Mortgage notes payable	$ 4,004,496,000		$ 4,111,487,000
Proceeds from (Repayments of) Debt	(206,340,000)
Mortgage loans assumed	106,600,000	99,131,000
Acquired Properties	5
Prepayment Penalty Charges	300,000
Unamortized Deferred Financing Costs Written Off	1,100,000
Various period of outstanding mortgage indebtedness	September 1, 2048
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.14%
Interest rate range to maximum	11.25%
Weighted Average Mortgage Debt Interest Rate	4.94%
Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	500,000,000
Credit enhanced debt [Member]
Debt Instrument [Line Items]
Mortgage notes payable	$ 362,200,000
Properties Acquired with Debt [Member]
Debt Instrument [Line Items]
Acquired Properties	1