Debt Mortgage Notes Payable (Details) (USD $)	9 Months Ended					9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Nov. 01, 2012 Term Loan [Member]	Sep. 30, 2012 Credit enhanced debt [Member]	Sep. 30, 2012 Properties Acquired with Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 500,000,000
Mortgage notes payable	3,948,115,000		4,111,487,000		362,200,000
Proceeds from (Repayments of) Debt	(291,000,000)
Mortgage loans assumed	137,644,000	99,131,000
Acquired Properties	9					2
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	0.16%
Interest rate range to maximum	11.25%
Weighted Average Mortgage Debt Interest Rate	4.95%