Assets Held for Sale and Discontinued Operations (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011 properties	Jun. 30, 2010 properties	Jun. 30, 2011 properties sqft	Jun. 30, 2010 properties	Dec. 31, 2010 sqft properties
Income Attributable To Discontinued Operations Details [Abstract]
Rental income	$ 2,959,000	$ 42,044,000	$ 12,913,000	$ 85,332,000
Rental expenses	12,000	(11,306,000)	(3,368,000)	(22,968,000)
Depreciation and amortization	(19,000)	(10,616,000)	(475,000)	(21,790,000)
Income attributable to disposed properties	2,952,000	20,122,000	9,070,000	40,574,000
Net gains recognized on dispositions of properties and other real estate investments	10,834,000	979,000	14,710,000	9,978,000
Impairment charges	(2,659,000)	0	(2,659,000)	0
Income tax on dispositions	0	0	1,916,000	851,000
Total discontinued operations attributable to the Parent Company and the Operating Partnership	11,127,000	21,101,000	19,205,000	49,701,000
Schedule Of Disposal Groups Including Discontinued Operations Income Statement Balance Sheet And Additional Disclosures Details [Abstract]
Number of properties	5	1	38	9
Net proceeds from dispositions	176,213,000	3,753,000	567,990,000	17,441,000
Net gains from dispositions	$ 8,175,000	$ 979,000	$ 10,135,000	$ 9,127,000
Discontinued Operations (Textuals) [Abstract]
Number of properties sold to third parties			38		205
Total square feet sold to third parties			2,800,000		25,400,000
Development Assets Sold To Third Parties					2