Debt (Summary of Debt) (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Mortgages payable	$ 8,988,843,000	$ 8,331,993,000
Senior unsecured notes	1,791,814,000	1,350,855,000
Revolving credit facility debt	88,138,000	295,870,000
Notes And Mortgages Payable Fixed Rate [Member]
Debt Instrument [Line Items]
Mortgages payable	7,623,049,000	7,563,133,000
Debt Instrument, Interest Rate at Period End (in percentage)	4.48%
Notes And Mortgages Payable Variable Rate [Member]
Debt Instrument [Line Items]
Mortgages payable	1,365,794,000	768,860,000
Debt Instrument, Interest Rate at Period End (in percentage)	2.31%
Total Notes And Mortgages Payable [Member]
Debt Instrument [Line Items]
Mortgages payable	8,988,843,000	8,331,993,000
Debt Instrument, Interest Rate at Period End (in percentage)	4.15%
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes	1,791,814,000	1,350,855,000
Debt Instrument, Interest Rate at Period End (in percentage)	4.88%
Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Revolving credit facility debt	88,138,000	295,870,000
Debt Instrument, Interest Rate at Period End (in percentage)	1.30%
Unsecured Debt [Member]
Debt Instrument [Line Items]
Unsecured Debt And Revolving Credit Facility	$ 1,879,952,000	$ 1,646,725,000
Debt Instrument, Interest Rate at Period End (in percentage)	4.71%

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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