DEBT - Narrative (Details)											1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 18, 2019 USD ($)	May 16, 2019 USD ($) 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 ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	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															1,225,000		1,225,000			876,000
Loss on early extinguishment of debt															0	$ 0	29,982,000	$ 0
Proceeds from notes payable																	0	38,200,000
Restricted cash															11,507,000	$ 21,524,000	11,507,000	21,524,000		22,512,000
Number of swaps terminated | swap				2			1												7
Notional amount of terminated swaps				$ 120,000,000			$ 50,000,000
Financing Receivable, Revolving																	74,000,000	0
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															552,000		552,000			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															2,232,000		$ 2,232,000			$ 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															0		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															$ 68,500,000		$ 68,500,000			$ 130,000,000
Remaining borrowing capacity															$ 175,100,000		$ 175,100,000			$ 91,000,000
Revolving Credit Facility | Subsequent Event | Line of Credit
Debt
Financing Receivable, Revolving	$ 76,500,000