ACQUISITIONS AND DISPOSITIONS - Carrying Value of Assets and Liabilities at Time of Sale and Held for Sale (Details) - USD ($) $ in Thousands			1 Months Ended	9 Months Ended
	Mar. 14, 2019	Mar. 01, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	May 16, 2019	Dec. 31, 2018
Disposal Group
Ownership sold, percentage				100.00%
Assets
Total assets, net				$ 0			$ 22,175
Liabilities
Total liabilities				0			28,766
Deferred loan costs				1,704
Debt, accumulated amortization				576
Payment of mortgages payable				46,000	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Assets
Investments in real estate, net				476,532
Deferred rent receivable and charges, net				55,297
Other intangible assets, net				316
Other assets				4,096
Total assets, net				536,241
Liabilities
Debt, net				318,072
Total liabilities				$ 318,072
Disposal Group, Held-for-sale, Not Discontinued Operations
Assets
Investments in real estate, net							17,123
Cash and cash equivalents							755
Accounts receivable, net							41
Deferred rent receivable and charges, net							4,009
Other intangible assets, net							220
Other assets							27
Total assets, net							22,175
Liabilities
Debt, net							28,018
Accounts payable and accrued expenses							370
Due to related parties							81
Other liabilities							297
Total liabilities							28,766
Deferred loan costs							243
Debt, accumulated amortization							61
Investments in real estate							24,832
Investments in real estate, accumulated depreciation							7,709
Deferred rent receivable							2,909
Deferred leasing costs							1,669
Deferred leasing costs, accumulated amortization							569
Debt, gross							28,200
Acquired In-Place Leases | Disposal Group, Held-for-sale, Not Discontinued Operations
Assets
Other intangible assets, net							1,778
Liabilities
Other intangible assets, accumulated amortization							$ 1,558
Office Property, San Fransisco California | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Outstanding amount	$ 28,200
Office Property, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Liabilities
Payment of mortgages payable		$ 46,000
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
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