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Property (Narrative) (Details)
$ in Thousands
9 Months Ended 12 Months Ended
Feb. 12, 2016
USD ($)
center
Sep. 30, 2016
USD ($)
property
center
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Business Acquisition [Line Items]        
Number of centers sold 1 7    
Proceeds from sale and disposition of property   $ 181,600    
Gain on sale of property   $ 68,298 $ 43,917 $ 1,000
Number of acquired centers | property   3    
Gross acquisition purchase price   $ 443,700    
Investment in new development   $ 19,600    
Number of centers classified as held for sale | property   1    
Center held for sale   $ 9,100    
Series of Individually Immaterial Business Acquisitions [Member]        
Business Acquisition [Line Items]        
Number of acquired centers | center   3    
Business acquisition, percentage of voting interests acquired   50.00%    
Property acquired   $ 432,205    
Business Combination Achieved in Stages [Member]        
Business Acquisition [Line Items]        
Number of acquired centers | center 2      
Business acquisition, percentage of voting interests acquired 49.00%      
Property acquired $ 58,665