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REAL ESTATE PROPERTIES (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Real Estate Properties [Line Items]      
Depreciation Expense During the Period $ 16,634,000 $ 15,666,000  
Real Estate Properties      
Land 320,555,000   $ 308,931,000
Building and building improvements 1,506,363,000   1,435,309,000
Tenant and other improvements 380,816,000   368,833,000
Development 244,313,000   293,908,000
Real estate and development properties 2,452,047,000   2,406,981,000
Less accumulated depreciation (710,875,000)   (694,250,000)
Real estate, net $ 1,741,172,000   $ 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