REAL ESTATE HELD FOR INVESTMENT - Assets Concentration Risk (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) ft² $ / ft²
Concentration Risk [Line Items]
Total Real Estate, Net	$ 139,703
Commonwealth Building | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	224,122
Total Real Estate, Net	$ 65,159
Percentage of Total Assets	44.20%
Annualized Base Rent	$ 5,585
Average Annualized Base Rent per sq. ft. | $ / ft²	28.60
Occupancy	87.10%
The Offices at Greenhouse | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	203,284
Total Real Estate, Net	$ 37,917
Percentage of Total Assets	25.70%
Annualized Base Rent	$ 4,206
Average Annualized Base Rent per sq. ft. | $ / ft²	20.69
Occupancy	100.00%
Institute Property | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	155,385
Total Real Estate, Net	$ 36,627
Percentage of Total Assets	24.90%
Annualized Base Rent	$ 3,748
Average Annualized Base Rent per sq. ft. | $ / ft²	27.78
Occupancy	86.80%