REAL ESTATE HELD FOR INVESTMENT (Schedule of Real Estate Investments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items]
Total Real Estate at Cost	$ 1,170,444	$ 1,164,690
Accumulated Depreciation and Amortization	(179,904)	(169,443)
Total real estate held for investment, net	990,540	$ 995,247
Office Properties
Real Estate Properties [Line Items]
Total Real Estate at Cost	1,170,444
Accumulated Depreciation and Amortization	(179,904)
Total real estate held for investment, net	$ 990,540
Number of real estate properties | property	8
Office Buildings, Campus
Real Estate Properties [Line Items]
Number of real estate properties | property	8
Office Buildings, Campus | Held-for-sale
Real Estate Properties [Line Items]
Number of real estate properties | property	2
100 & 200 Campus Drive Buildings | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 09, 2008
Total Real Estate at Cost	$ 149,908
Accumulated Depreciation and Amortization	(12,287)
Total real estate held for investment, net	$ 137,621
300-600 Campus Drive Buildings | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Oct. 10, 2008
Total Real Estate at Cost	$ 162,300
Accumulated Depreciation and Amortization	(17,902)
Total real estate held for investment, net	$ 144,398
Willow Oaks Corporate Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Aug. 26, 2009
Total Real Estate at Cost	$ 104,886
Accumulated Depreciation and Amortization	(19,793)
Total real estate held for investment, net	$ 85,093
Pierre Laclede Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Feb. 04, 2010
Total Real Estate at Cost	$ 81,221
Accumulated Depreciation and Amortization	(10,645)
Total real estate held for investment, net	$ 70,576
Union Bank Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 15, 2010
Total Real Estate at Cost	$ 185,549
Accumulated Depreciation and Amortization	(24,188)
Total real estate held for investment, net	$ 161,361
Emerald View at Vista Center | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 09, 2010
Total Real Estate at Cost	$ 31,622
Accumulated Depreciation and Amortization	(7,121)
Total real estate held for investment, net	$ 24,501
Granite Tower | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Dec. 16, 2010
Total Real Estate at Cost	$ 154,590
Accumulated Depreciation and Amortization	(44,474)
Total real estate held for investment, net	$ 110,116
Fountainhead Plaza | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Sep. 13, 2011
Total Real Estate at Cost	$ 119,384
Accumulated Depreciation and Amortization	(19,150)
Total real estate held for investment, net	$ 100,234
Corporate Technology Centre | Office Properties
Real Estate Properties [Line Items]
Date Acquired	Mar. 28, 2013
Total Real Estate at Cost	$ 180,984
Accumulated Depreciation and Amortization	(24,344)
Total real estate held for investment, net	$ 156,640