REAL ESTATE HELD FOR INVESTMENT - Schedule of Real Estate Investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 162,812	$ 173,131
Accumulated Depreciation and Amortization	(23,109)	$ (22,688)
Total Real Estate, Net	$ 139,703
Commonwealth Building | Office Building
Real Estate Properties [Line Items]
Date Acquired	Jun. 30, 2016
Total Real Estate at Cost	$ 78,182
Accumulated Depreciation and Amortization	(13,023)
Total Real Estate, Net	$ 65,159
The Offices at Greenhouse | Office Building
Real Estate Properties [Line Items]
Date Acquired	Nov. 14, 2016
Total Real Estate at Cost	$ 47,165
Accumulated Depreciation and Amortization	(9,248)
Total Real Estate, Net	$ 37,917
Institute Property | Office Building
Real Estate Properties [Line Items]
Date Acquired	Nov. 09, 2017
Total Real Estate at Cost	$ 37,465
Accumulated Depreciation and Amortization	(838)
Total Real Estate, Net	$ 36,627