REAL ESTATE SALES (Details) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 17, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) property	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) property	Jul. 10, 2013 a ft² property
Real Estate Properties [Line Items]
Total real estate, cost		$ 860,828		$ 860,828		$ 539,376
Accumulated depreciation and amortization		56,749		56,749		$ 37,154
Office Properties | Central Building
Real Estate Properties [Line Items]
Number of real estate properties acquired | property							1
Net rentable area | ft²							191,784
Area of land | a							0.6
Disposed of by Sale
Revenues
Rental income		185	$ 17,129	2,255	$ 53,262
Tenant reimbursements and other operating income		16	5,028	267	15,511
Total revenues		201	22,157	2,522	68,773
Expenses
Operating, maintenance, and management		20	6,702	453	19,453
Real estate taxes and insurance		(137)	3,170	150	9,649
Asset management fees to affiliate		79	1,371	351	4,197
Depreciation and amortization		0	8,064	483	24,296
Interest expense		83	3,843	623	10,843
Total expenses		45	$ 23,150	$ 2,060	$ 68,438
Disposed of by Sale | Central Building
Real Estate Properties [Line Items]
Sale price	$ 67,500
Total real estate, cost	32,600
Accumulated depreciation and amortization	5,600
Gain on disposal	$ 32,100
Disposed of by Sale | Office Properties
Real Estate Properties [Line Items]
Number of real estate properties disposed | property				1		12
Held-for-sale
Assets and Liabilities
Real estate, cost		0		$ 0		$ 35,308
Accumulated depreciation and amortization		0		0		(4,663)
Real estate, net		0		0		30,645
Other assets		0		0		1,965
Total assets related to real estate held for sale		0		0		32,610
Notes payable, net		0		0		27,515
Total liabilities related to real estate held for sale		$ 0		$ 0		$ 27,515