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Significant Acquisitions and Dispositions (Narrative) (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2015
USD ($)
buildings
Sep. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
buildings
Sep. 30, 2014
USD ($)
Jun. 30, 2014
USD ($)
Mar. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
buildings
Dec. 31, 2014
USD ($)
buildings
Dec. 31, 2013
USD ($)
Business Acquisition [Line Items]                      
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life                 9 years 2 months 1 day 9 years 13 days  
Number Of Real Estate Properties Acquired | buildings         5         5  
Rental and related revenue $ 198,516,000 $ 200,938,000 $ 201,996,000 $ 214,615,000 $ 206,859,000 $ 202,067,000 $ 204,780,000 $ 208,645,000 $ 816,065,000 $ 822,351,000 $ 762,164,000
Operating Income (Loss)                 448,396,000 411,068,000 267,235,000
Business Combination, Acquisition Related Costs                 8,500,000 1,100,000 4,100,000
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain                 $ 0 $ 0 $ 962,000
Industrial [Member]                      
Business Acquisition [Line Items]                      
Number Of Real Estate Properties Acquired | buildings 2               2    
Other 2015 Acquisitions [Member]                      
Business Acquisition [Line Items]                      
Rental and related revenue                 $ 988,000    
Operating Income (Loss)                 $ 135,000    
Industrial [Member]                      
Business Acquisition [Line Items]                      
Number Of Real Estate Properties Acquired | buildings         4         4  
Medical Office [Member]                      
Business Acquisition [Line Items]                      
Number Of Real Estate Properties Acquired | buildings         1         1