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Schedule III-Real Estate and Accumulated Depreciation (Details 2) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2007
Changes in total real estate assets        
Balances, beginning of year $ 6,908,507 $ 6,697,259 $ 7,355,703 $ 7,078,802
Additions 784,717 239,362 241,025  
Dispositions and retirements (203,489) (28,114) (899,469)  
Balances, end of year 7,489,735 6,908,507 6,697,259 7,078,802
Changes in accumulated depreciation        
Balances, beginning of year 1,234,380 1,039,320 984,384  
Additions 223,630 206,913 224,279  
Dispositions and retirements (47,318) (11,853) (169,343)  
Balances, end of year $ 1,410,692 $ 1,234,380 $ 1,039,320  
Buildings and improvements | Minimum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 5      
Buildings and improvements | Maximum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 40      
Tenant improvements | Minimum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 5      
Tenant improvements | Maximum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 7      
Equipment and furnishings | Minimum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 5      
Equipment and furnishings | Maximum
        
REAL ESTATE AND ACCUMULATED DEPRECIATION        
Estimated useful lives of assets (in years) 7