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Property: (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Property, Plant and Equipment [Abstract]      
Land $ 1,894,717 $ 2,242,291  
Buildings and improvements 7,752,892 9,479,337  
Tenant improvements 637,355 600,436  
Equipment and furnishings 169,841 152,554  
Construction in progress 234,851 303,264  
Total 10,689,656 12,777,882  
Less accumulated depreciation (1,892,744) (1,709,992)  
Property, net 8,796,912 11,067,890  
Depreciation expense $ 354,977 $ 289,178 $ 269,790