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REAL ESTATE PROPERTIES (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Real Estate Properties [Line Items]      
Depreciation Expense During the Period $ 15,666,000 $ 14,838,000  
Real Estate Properties      
Land 301,909,000   $ 301,435,000
Building and building improvements 1,406,871,000   1,393,688,000
Tenant and other improvements 355,543,000   353,884,000
Development 161,659,000   170,441,000
Real estate and development properties 2,225,982,000   2,219,448,000
Less accumulated depreciation (665,010,000)   (657,454,000)
Real estate, net $ 1,560,972,000   $ 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