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REAL ESTATE PROPERTIES (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Real Estate Properties [Line Items]          
Depreciation Expense During the Period $ 17,323,000 $ 15,855,000 $ 51,070,000 $ 47,273,000  
Real Estate Properties          
Land 341,141,000   341,141,000   $ 308,931,000
Building and building improvements 1,569,982,000   1,569,982,000   1,435,309,000
Tenant and other improvements 394,894,000   394,894,000   368,833,000
Development 204,197,000   204,197,000   293,908,000
Real estate and development properties 2,510,214,000   2,510,214,000   2,406,981,000
Less accumulated depreciation (732,566,000)   (732,566,000)   (694,250,000)
Real estate, net $ 1,777,648,000   $ 1,777,648,000   $ 1,712,731,000
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