CONCENTRATIONS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 property
Office properties
Concentrations
Number of real estate properties owned	20	20	21
Multifamily properties
Concentrations
Number of real estate properties owned	5	5	5
Hotel properties
Concentrations
Number of real estate properties owned	1	3	3
Parking garages
Concentrations
Number of real estate properties owned	3	3	3
Development site
Concentrations
Number of real estate properties owned	2	2	1
Revenues | Tenant revenue concentrations | Governmental Tenants
Concentrations
Concentration risk, percent	19.90%	22.70%	24.70%
Amount due from Governmental Tenants | $	$ 8,339	$ 7,968
Revenues | Geographical concentrations
Concentrations
Concentration risk, percent	100.00%	100.00%	100.00%
Revenues | Geographical concentrations | California
Concentrations
Concentration risk, percent	63.40%	62.20%	60.40%
Revenues | Geographical concentrations | Washington, D.C.
Concentrations
Concentration risk, percent	21.30%	24.20%	24.70%
Revenues | Geographical concentrations | Texas
Concentrations
Concentration risk, percent	8.20%	7.80%	7.70%
Revenues | Geographical concentrations | North Carolina
Concentrations
Concentration risk, percent	5.20%	4.60%	5.10%
Revenues | Geographical concentrations | New York
Concentrations
Concentration risk, percent	1.90%	1.20%	2.10%
Real estate investments | Geographical concentrations
Concentrations
Concentration risk, percent	100.00%	100.00%
Real estate investments | Geographical concentrations | California
Concentrations
Concentration risk, percent	50.80%	52.60%
Real estate investments | Geographical concentrations | Washington, D.C.
Concentrations
Concentration risk, percent	32.30%	31.10%
Real estate investments | Geographical concentrations | Texas
Concentrations
Concentration risk, percent	7.70%	7.40%
Real estate investments | Geographical concentrations | North Carolina
Concentrations
Concentration risk, percent	5.50%	5.30%
Real estate investments | Geographical concentrations | New York
Concentrations
Concentration risk, percent	3.70%	3.60%