Significant Acquisitions and Dispositions Dispositions (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dispositions Disclosures [Line Items]
Proceeds from land and depreciated property sales, net	$ 138,118,000	$ 1,572,093,000	$ 499,520,000
Blackstone Real Estate Partners [Member]
Dispositions Disclosures [Line Items]
Number of Real Estate Properties Sold		79
Square Footage Of Real Estate Properties Acquired Or Disposed		9,800,000
Sales Price		1,060,000,000
Proceeds from Sales of Assets, Investing Activities		1,020,000,000
Sale of Real Estate, Debt Assumed by Buyer		24,900,000
Office Building [Member]
Dispositions Disclosures [Line Items]
Proceeds from land and depreciated property sales, net		273,700,000	139,100,000
Number of Real Estate Properties Sold		13	7
Square Footage Of Real Estate Properties Acquired Or Disposed		2,000,000	1,000,000
Sales Price		$ 342,800,000	$ 173,900,000
Percentage Share of Proceeds From Property Disposition		80.00%	80.00%
Percentage Retained Ownership In Property Disposition		20.00%	20.00%