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REAL ESTATE PROPERTIES (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Real Estate Properties [Line Items]          
Depreciation Expense During the Period $ 15,752,000 $ 14,720,000 $ 31,418,000 $ 29,558,000  
Real Estate Properties          
Land 297,276,000   297,276,000   $ 301,435,000
Building and building improvements 1,391,114,000   1,391,114,000   1,393,688,000
Tenant and other improvements 354,644,000   354,644,000   353,884,000
Development 166,758,000   166,758,000   170,441,000
Real estate and development properties 2,209,792,000   2,209,792,000   2,219,448,000
Less accumulated depreciation (664,093,000)   (664,093,000)   (657,454,000)
Real estate, net $ 1,545,699,000   $ 1,545,699,000   $ 1,561,994,000
Building [Member]          
Real Estate Properties [Line Items]          
Property, Plant and Equipment, Useful Life     40 years    
Minimum [Member] | Improvements [Member]          
Real Estate Properties [Line Items]          
Property, Plant and Equipment, Useful Life     3 years    
Maximum [Member] | Improvements [Member]          
Real Estate Properties [Line Items]          
Property, Plant and Equipment, Useful Life     15 years