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Schedule III-Real Estate and Accumulated Depreciation (Details 2) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Changes in total real estate assets      
Balances, beginning of year $ 7,489,735 $ 6,908,507 $ 6,697,259
Additions 1,909,530 784,717 239,362
Dispositions and retirements (386,559) (203,489) (28,114)
Balances, end of year 8,070,963 7,489,735 6,908,507
Changes in accumulated depreciation      
Balances, beginning of year 1,410,692 1,234,380 1,039,320
Additions 223,630 241,231 206,913
Dispositions and retirements (47,318) (47,318) (11,853)
Balances, end of year $ 1,587,004 $ 1,410,692 $ 1,234,380
Buildings and improvements | Minimum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 5 years    
Buildings and improvements | Maximum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 40 years    
Tenant improvements | Minimum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 5 years    
Tenant improvements | Maximum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 7 years    
Equipment and furnishings | Minimum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 5 years    
Equipment and furnishings | Maximum
      
REAL ESTATE AND ACCUMULATED DEPRECIATION      
Estimated useful lives of assets 7 years