ACQUISITIONS AND DISPOSITIONS - Carrying Value of Assets and Liabilities at Time of Sale and Held for Sale (Details)			1 Months Ended		3 Months Ended			6 Months Ended
	Mar. 14, 2019 USD ($)	Mar. 01, 2019 USD ($)	Jul. 31, 2019 USD ($)	Mar. 31, 2019	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) property	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	May 16, 2019 USD ($)	Dec. 31, 2018 USD ($)
Disposal Group
Ownership sold, percentage								100.00%
Assets
Total assets, net					$ 178,927,000			$ 178,927,000			$ 22,175,000
Liabilities
Total liabilities					3,245,000			3,245,000			28,766,000
Deferred loan costs					1,704,000			1,704,000
Debt, accumulated amortization					576,000			576,000
Payment of mortgages payable								46,000,000	$ 0
Impairment of real estate					2,800,000	$ 66,200,000	$ 0	69,000,000	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Assets
Investments in real estate, net					318,918,000			318,918,000
Deferred rent receivable and charges, net					41,280,000			41,280,000
Other intangible assets, net					316,000			316,000
Total assets, net					360,514,000			360,514,000
Liabilities
Debt, net					318,072,000			318,072,000
Total liabilities					318,072,000			318,072,000
Disposal Group, Held-for-sale, Not Discontinued Operations
Assets
Investments in real estate, net					157,608,000			157,608,000			17,123,000
Cash and cash equivalents					2,050,000			2,050,000			755,000
Accounts receivable, net					1,075,000			1,075,000			41,000
Deferred rent receivable and charges, net					13,992,000			13,992,000			4,009,000
Other intangible assets, net					0			0			220,000
Other assets					4,202,000			4,202,000			27,000
Total assets, net					178,927,000			178,927,000			22,175,000
Liabilities
Debt, net					0			0			28,018,000
Accounts payable and accrued expenses					2,160,000			2,160,000			370,000
Due to related parties					518,000			518,000			81,000
Other liabilities					567,000			567,000			297,000
Total liabilities					3,245,000			3,245,000			28,766,000
Deferred loan costs											243,000
Debt, accumulated amortization											61,000
Investments in real estate					230,523,000			230,523,000			24,832,000
Investments in real estate, accumulated depreciation					72,915,000			72,915,000			7,709,000
Deferred rent receivable					9,640,000			9,640,000			2,909,000
Deferred leasing costs					11,165,000			11,165,000			1,669,000
Deferred leasing costs, accumulated amortization					6,813,000			6,813,000			569,000
Lease inducements					8,966,000			8,966,000
Lease inducements, accumulated amortization					$ 4,870,000			$ 4,870,000
Debt, gross											28,200,000
Acquired In-Place Leases | Disposal Group, Held-for-sale, Not Discontinued Operations
Assets
Other intangible assets, net											1,778,000
Liabilities
Other intangible assets, accumulated amortization											$ 1,558,000
Office Property, San Fransisco California | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Outstanding amount	$ 28,200,000
Office Property, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Payment of mortgages payable		$ 46,000,000
260 Townsend Street, San Francisco, CA
Disposal Group
Ownership sold, percentage				100.00%
Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026 | Properties Used As Collateral For Loans | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Defeased amount		$ 205,500,000
Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026 | 1333 Broadway, Oakland, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Defeased amount										$ 39,500,000
Land and Building
Liabilities
Ownership percentage to be sold in potential sale						100.00%
Building
Liabilities
Number of real estate investment property sold in potential sale | property						2
Land
Liabilities
Number of real estate investment property sold in potential sale | property						1
Forecast | Subsequent Event | Two Office Properties And One Developmental Site, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Gain (loss) on sale			$ 200,000