Unsecured Line Of Credit (Details) (USD $)	9 Months Ended
	Sep. 30, 2011
Line of Credit Facility [Line Items]
Line of credit facility, extension fee	0.20%
Provision for extension, years	1
Line of credit facility, facility fee percentage	0.225%
Frequency of payment	quarterly
Maximum leverage ratio	60.00%
Leverage ratio maximum increasing limit	65.00%
Leverage ratio reduction limit	60.00%
Leverage ratio reduction period (years)	1
Maximum secured debt leverage ratio	55.00%
Fixed charge coverage ratio, minimum	1.40
Maximum unsecured debt leverage ratio	60.00%
Unsecured debt leverage ratio maximum increasing limit	65.00%
Unsecured debt leverage ratio reduction limit	60.00%
Unsecured debt leverage ratio reduction period (years)	1
Unsecured debt interest coverage ratio, minimum	1.75
Line of credit facility, amount outstanding	$ 0
Repayments of unsecured line of credit	453,300,000
Minimum net worth requirement	3,500,000,000
Potential Borrowing Capacity Subject To Syndication [Member]
Line of Credit Facility [Line Items]
Current lenders' total commitment	1,000,000,000
Prior To Amendment [Member]
Line of Credit Facility [Line Items]
Current lenders' total commitment	1,000,000,000
Line of credit facility, expiration date	August 3, 2011
Basis spread on variable rate basis	0.475%
Description of variable rate basis	Eurodollar plus 0.475% per annum
After The Amendment [Member]
Line of Credit Facility [Line Items]
Current lenders' total commitment	$ 750,000,000
Line of credit facility, expiration date	June 24, 2014
Basis spread on variable rate basis	1.225%
Description of variable rate basis	Eurodollar plus 1.225% per annum