Significant Acquisitions and Dispositions Dispositions (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Dispositions Disclosures [Line Items]
Proceeds From Land And Depreciated Property Sales Net	$ 1,572,093,000	$ 499,520,000	$ 256,330,000
Suburban Office Properties [Member]
Dispositions Disclosures [Line Items]
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 of Real Estate Properties	342,800,000	173,900,000
Percentage Share of Proceeds From Property Disposition	80.00%	80.00%
Proceeds from Sales of Assets, Investing Activities	273,700,000	139,100,000
Percentage Retained Ownership In Property Disposition	20.00%	20.00%
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 of Real Estate Properties	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