Indebtedness - Credit Facility (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	$ 139,400,000	$ 0
Line of Credit Facility, Commitment Fee Percentage	0.20%
Total Assets for Covenant Ratio	4,989,673,000	5,064,799,000
New Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000
Line of Credit Facility, Expiration Date	November 2015
Length of Maturity Extension	1
Competitive Bid Option % of Facility Amount	50.00%
Line of Credit Facility, Amount Outstanding	139,400,000
Line of Credit Facility, Interest Rate at Period End	1.48%
New Credit Facility [Member] | Total Debt to Total Assets Ratio [Member]
Line of Credit Facility [Line Items]
Borrowed Funds for Covenant Ratio	0.60
Total Assets for Covenant Ratio	1
New Credit Facility [Member] | EBITDA to Fixed Charges Ratio [Member]
Line of Credit Facility [Line Items]
Earnings Before Interest, Taxes, Depreciation and Amortization for Covenant Ratio	1.50
Fixed Charges for Covenant Ratio	1
New Credit Facility [Member] | Unsecured Debt to Unencumbered Asset Ratio [Member]
Line of Credit Facility [Line Items]
Unsecured Debt to Unencumbered Asset Value for Debt Covenant	60.00%
New Credit Facility [Member] | Unencumbered Net Operating Income to Unsecured Interest Expense [Member]
Line of Credit Facility [Line Items]
Unencumbered Net Operating Income to Unsecured Interest Expense Value for Debt Covenant	200.00%
Old Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000
Line of Credit Facility, Expiration Date	November 2013