Mortgage Notes Payable (Details) (USD $)	6 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Mortgage notes payable	$ 4,352,372,000	$ 4,762,896,000
Interest rate range from minimum	0.69%
Interest rate range to maximum	7.57%
Mortgage Notes Payable (Textuals) [Abstract]
Scheduled principal repayments	640,800,000
Write-off of unamortized deferred financing costs	2,100,000
Obtained new mortgage loans	135,200,000
Assumed mortgage debt	99,100,000
Properties acquired	3
Secured Debt [Member]
Debt Instrument [Line Items]
Mortgage notes payable	$ 446,500,000
Company range of interest rates on outstanding mortgage indebtedness at various dates through September 1, 2048 [Member]
Debt Instrument [Line Items]
Interest rate range from minimum	0.09%
Interest rate range to maximum	11.25%
Average interest rate	4.81%

Mortgage Notes Payable.

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Total number of property or properties acquired from unaffiliated third parties during the period.

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Represents charges to interest expense related to the extinguishment of borrowing before maturity.

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When presenting a range of interest rates, the highest stated (contractual) rate for funds borrowed under the debt agreement as of the balance sheet date.

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When presenting a range of interest rates, the lowest stated (contractual) rate for funds borrowed under the debt agreement as of the balance sheet date.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of debt outstanding by type or by instrument at that time.

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Total amount of new mortgage loans on real estate during the reporting period.

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The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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The net cash inflow or outflow in aggregate debt due to repayments and proceeds from additional borrowings.

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Carrying value as of the balance sheet date , including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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