DEBT - Mortgage Payable and Loan Backed Notes Narrative (Details)						1 Months Ended	3 Months Ended		9 Months Ended
	May 16, 2019 USD ($) mortgageLoan	Mar. 14, 2019 USD ($) mortgageLoan	Mar. 01, 2019 USD ($)	May 30, 2018 USD ($)	Sep. 21, 2017 USD ($) mortgageLoan	Jun. 30, 2016 USD ($) agreement	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Deferred loan costs							$ 3,466,000		$ 3,466,000		$ 4,535,000
Deferred loan costs, accumulated amortization							890,000		890,000		1,494,000
Loss on early extinguishment of debt							281,000	$ 0	281,000	$ 29,982,000
Prepaid of mortgage loan									0	46,000,000
Prepayment penalty									0	5,660,000
Restricted cash							9,877,000	11,507,000	9,877,000	11,507,000	12,146,000
Disposal Group, Held-for-sale, Not Discontinued Operations | 7083 Hollywood Boulevard, Los Angeles, California
Debt Instrument [Line Items]
Number of mortgage loans assumed by buyer | mortgageLoan					1
Outstanding principal amount					$ 21,700,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Office Property, San Fransisco California
Debt Instrument [Line Items]
Number of mortgage loans assumed by buyer | mortgageLoan		1
Outstanding principal amount		$ 28,200,000
Loss on early extinguishment of debt								0		178,000
Write off of deferred loan costs										243,000
Write off of accumulated amortization										65,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Office Property, Washington, D.C.
Debt Instrument [Line Items]
Loss on early extinguishment of debt								0		5,603,000
Write off of deferred loan costs			$ 537,000
Write off of accumulated amortization			259,000
Prepaid of mortgage loan			46,000,000
Prepayment penalty			5,325,000
Mortgages Payable
Debt Instrument [Line Items]
Number of mortgage loan agreements entered into | agreement						6
Aggregate principal amount						$ 392,000,000
Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026 | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Properties Used As Collateral For Loans
Debt Instrument [Line Items]
Defeased amount			205,500,000
Cash outlay required for defeasance			224,086,000
Loss on early extinguishment of debt								0		19,290,000
Write off of deferred loan costs			637,000
Write off of accumulated amortization			170,000
Transaction costs			$ 237,000
Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026 | Disposal Group, Disposed of by Sale, Not Discontinued Operations | 1333 Broadway, Oakland, CA
Debt Instrument [Line Items]
Defeased amount	$ 39,500,000
Cash outlay required for defeasance	44,108,000
Loss on early extinguishment of debt								$ 0		$ 4,911,000
Write off of deferred loan costs	287,000
Write off of accumulated amortization	82,000
Transaction costs	$ 98,000
Number of mortgage loans defeased | mortgageLoan	1
Loan Backed Notes
Debt Instrument [Line Items]
Proceeds from notes payable				$ 38,200,000
Weighted average life				2 years
Restricted cash							$ 1,113,000		$ 1,113,000		$ 3,306,000
Loan Backed Notes | LIBOR
Debt Instrument [Line Items]
Interest rate margin				1.40%
Loan Backed Notes | Prime Rate
Debt Instrument [Line Items]
Interest rate margin				(1.08%)