DEBT - Narrative (Details)										1 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended
	May 16, 2019 USD ($) mortgage_loan	Mar. 14, 2019 USD ($) mortgage_loan	Mar. 11, 2019 USD ($) swap	Mar. 01, 2019 USD ($)	Feb. 28, 2019 USD ($)	Dec. 28, 2018 USD ($) swap	Oct. 30, 2018 USD ($)	May 30, 2018 USD ($)	Sep. 21, 2017 USD ($) mortgage_loan	Oct. 31, 2018 USD ($) extension	Jun. 30, 2016 USD ($) agreement	May 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) swap	Dec. 31, 2018 USD ($)	Nov. 02, 2015 USD ($)	Aug. 13, 2015 USD ($)
Debt
Deferred loan costs														$ 4,535,000		$ 4,535,000			$ 5,994,000
Deferred loan costs, accumulated amortization														951,000		951,000			876,000
Loss on early extinguishment of debt														4,911,000	$ 0	29,982,000	$ 0
Proceeds from notes payable																0	38,200,000
Restricted cash														10,824,000	$ 22,800,000	10,824,000	22,800,000		22,512,000
Number of swaps terminated | swap			2			1												7
Notional amount of terminated swaps			$ 120,000,000			$ 50,000,000
Payment of mortgages payable																46,000,000	0
Prepayment penalties and other payments for early extinguishment of debt																5,660,000	$ 0
Accrued interest and unused commitment fee payable														571,000		571,000			1,574,000
Unsecured credit facility entered into in September 2014
Debt
Maximum borrowing capacity													$ 850,000,000
Unsecured credit facility entered into in September 2014, revolver
Debt
Maximum borrowing capacity													$ 450,000,000
Unsecured credit facility entered into in September 2014, revolver | Minimum
Debt
Interest rate margin													0.20%
Unsecured credit facility entered into in September 2014, revolver | Maximum
Debt
Interest rate margin													1.00%
Unsecured credit facility entered into in September 2014, revolver | LIBOR | Minimum
Debt
Interest rate margin													1.20%
Unsecured credit facility entered into in September 2014, revolver | LIBOR | Maximum
Debt
Interest rate margin													2.00%
Unsecured credit facility entered into in September 2014, term loan
Debt
Maximum borrowing capacity													$ 325,000,000
Unsecured credit facility entered into in September 2014, term loan | LIBOR | Minimum
Debt
Interest rate margin													1.15%
Unsecured credit facility entered into in September 2014, term loan | LIBOR | Maximum
Debt
Interest rate margin													1.95%
Unsecured credit facility entered into in September 2014, term loan | Base rate | Minimum
Debt
Interest rate margin													0.15%
Unsecured credit facility entered into in September 2014, term loan | Base rate | Maximum
Debt
Interest rate margin													0.95%
Unsecured credit facility entered into in September 2014, delayed-draw term loan
Debt
Maximum borrowing capacity													$ 75,000,000
Unsecured Term Loan Facility Entered Into May 2015
Debt
Deferred loan costs, accumulated amortization																		$ 705,000
Maximum borrowing capacity												$ 385,000,000								$ 385,000,000
Interest rate												3.16%
Repayments of debt																		215,000,000
Write off of deferred loan costs							$ 1,872,000											$ 1,988,000
Repayments of long-term debt							170,000,000
Write off of accumulated amortization							1,064,000
Unsecured Term Loan Facility Entered Into May 2015 | LIBOR | Minimum
Debt
Interest rate margin												1.60%
Unsecured Term Loan Facility Entered Into May 2015 | LIBOR | Maximum
Debt
Interest rate margin												2.25%
Unsecured Term Loan Facility Entered Into May 2015 | Base rate | Minimum
Debt
Interest rate margin												0.60%
Unsecured Term Loan Facility Entered Into May 2015 | Base rate | Maximum
Debt
Interest rate margin												1.25%
Mortgages Payable
Debt
Number of mortgage loan agreements | agreement											6
Amount of loan											$ 392,000,000
Gross debt														97,100,000		97,100,000			388,100,000
Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026
Debt
Gross debt														97,100,000		97,100,000			342,100,000
Loan Backed Notes
Debt
Proceeds from notes payable								$ 38,200,000
Weight average life								2 years
Restricted cash														1,756,000		$ 1,756,000			$ 3,174,000
Loan Backed Notes | LIBOR
Debt
Interest rate margin								1.40%								1.40%
Loan Backed Notes | Prime Rate
Debt
Interest rate margin								1.08%								1.08%
Revolving Credit Facility
Debt
Maximum borrowing capacity										$ 250,000,000
Interest rate																			4.07%
Number of extensions | extension										1
Extension option, term										1 year
Proceeds from issuance of debt							$ 170,000,000
Revolving Credit Facility | Minimum
Debt
Unused commitment fee percentage										0.15%
Revolving Credit Facility | Maximum
Debt
Unused commitment fee percentage										0.25%
Revolving Credit Facility | LIBOR
Debt
Interest rate margin										1.55%
Revolving Credit Facility | Base rate
Debt
Interest rate margin										0.55%
7083 Hollywood Boulevard, Los Angeles, California | Disposal Group, Held-for-sale, Not Discontinued Operations
Debt
Outstanding amount									$ 21,700,000
Number of mortgage loans assumed by buyer | mortgage_loan									1
Properties Used As Collateral For Loans | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026
Debt
Write off of deferred loan costs				$ 637,000
Write off of accumulated amortization				170,000
Defeased amount				205,500,000
Cash outlay required for defeasance				224,086,000
Loss on early extinguishment of debt														0		$ 19,290,000
Transaction costs				237,000
Office Property, San Fransisco California | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Write off of deferred loan costs																243,000
Write off of accumulated amortization																65,000
Outstanding amount		$ 28,200,000
Loss on early extinguishment of debt														0		178,000
Number of mortgage loans assumed by buyer | mortgage_loan		1
1333 Broadway, Oakland, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Mortgage loan with a fixed interest of 4.14% per annum, due on July 1, 2026
Debt
Write off of deferred loan costs	$ 287,000
Write off of accumulated amortization	82,000
Defeased amount	39,500,000
Cash outlay required for defeasance	44,108,000
Loss on early extinguishment of debt														4,911,000		4,911,000
Transaction costs	$ 98,000
Number of mortgage loans defeased | mortgage_loan	1
Office Property, Washington, D.C. | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Write off of deferred loan costs				537,000
Write off of accumulated amortization				259,000
Loss on early extinguishment of debt														0		5,603,000
Payment of mortgages payable				46,000,000
Prepayment penalties and other payments for early extinguishment of debt				$ 5,325,000
Interest Rate Swap
Debt
Total notional amount																					$ 385,000,000
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedges
Debt
Total notional amount														$ 120,000,000		$ 120,000,000
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedges | Weighted Average
Debt
All-in rate														3.11%		3.11%
Revolving Credit Facility | Line of Credit
Debt
Repayments of debt			$ 120,000,000		$ 10,000,000	$ 40,000,000
Gross debt														$ 0		$ 0			$ 130,000,000
Remaining borrowing capacity														$ 210,000,000		$ 210,000,000			$ 91,000,000