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Property: (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Property:      
Land $ 1,723,388   $ 1,572,621
Buildings and improvements 6,715,582   6,417,674
Tenant improvements 534,354   496,203
Equipment and furnishings 150,559   149,959
Construction in progress 408,860   376,249
Total 9,532,743   9,012,706
Less accumulated depreciation (1,600,180)   (1,533,160)
Property, net 7,932,563   7,479,546
Depreciation expenses 71,098 57,322  
Write off of development cost 591 1,034  
Gain on sale of assets $ 5,419