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Property (Narrative) (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
property
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
property
Real Estate [Abstract]      
Number of centers sold | property 14    
Proceeds from sale and disposition of property $ 334,100    
Gain on sale of property 155,998 $ 47,987  
Investment in new development $ 40,300    
Number of centers classified as held for sale | property 2   3
Center held for sale $ 35,500   $ 78,700