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CONCENTRATIONS (Details)
12 Months Ended
Dec. 31, 2015
USD ($)
state
item
Dec. 31, 2014
USD ($)
item
Dec. 31, 2013
item
Concentrations      
Concentration risk (as a percent)     100.00%
Number of states in which real estate properties are owned | state 4    
Office properties      
Concentrations      
Number of real estate properties owned 20 21 19
Multifamily properties      
Concentrations      
Number of real estate properties owned 5 5 5
Hotel properties      
Concentrations      
Number of real estate properties owned 3 3 3
Parking garages      
Concentrations      
Number of real estate properties owned 3    
Parking lot      
Concentrations      
Number of real estate properties owned 1    
Development site      
Concentrations      
Number of real estate properties owned 1 1 1
Revenues | Tenant Revenue Concentrations | Governmental Tenants      
Concentrations      
Concentration risk (as a percent) 22.70% 24.70% 27.50%
Amount due from Governmental Tenants | $ $ 7,968,000 $ 7,168,000  
Revenues | Geographical concentrations      
Concentrations      
Concentration risk (as a percent) 100.00% 100.00%  
Revenues | Geographical concentrations | California      
Concentrations      
Concentration risk (as a percent) 62.20% 60.40% 57.10%
Revenues | Geographical concentrations | Washington, D.C.      
Concentrations      
Concentration risk (as a percent) 24.20% 24.70% 26.50%
Revenues | Geographical concentrations | Texas      
Concentrations      
Concentration risk (as a percent) 7.80% 7.70% 8.10%
Revenues | Geographical concentrations | North Carolina      
Concentrations      
Concentration risk (as a percent) 4.60% 5.10% 6.10%
Revenues | Geographical concentrations | New York      
Concentrations      
Concentration risk (as a percent) 1.20% 2.10% 2.20%
Real estate investments | Geographical concentrations      
Concentrations      
Concentration risk (as a percent) 100.00% 100.00%  
Real estate investments | Geographical concentrations | California      
Concentrations      
Concentration risk (as a percent) 52.60% 52.30%  
Real estate investments | Geographical concentrations | Washington, D.C.      
Concentrations      
Concentration risk (as a percent) 31.10% 31.20%  
Real estate investments | Geographical concentrations | Texas      
Concentrations      
Concentration risk (as a percent) 7.40% 7.40%  
Real estate investments | Geographical concentrations | North Carolina      
Concentrations      
Concentration risk (as a percent) 5.30% 5.50%  
Real estate investments | Geographical concentrations | New York      
Concentrations      
Concentration risk (as a percent) 3.60% 3.60%