DEBT - Narrative (Details)														1 Months Ended				3 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 ($)	May 30, 2018 USD ($)	Dec. 15, 2017 USD ($)	Sep. 21, 2017 USD ($) mortgage_loan	Jun. 23, 2017 USD ($)	May 30, 2017 USD ($) property mortgage_loan	Mar. 28, 2017 USD ($)	Oct. 31, 2018 USD ($) extension_option	Jun. 30, 2016 USD ($) agreement	May 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) swap	Aug. 13, 2015 USD ($)
Debt
Deferred loan costs, gross																		$ 4,535,000				$ 5,994,000				$ 4,535,000	$ 5,994,000
Deferred loan costs, accumulated amortization																		1,494,000				876,000				1,494,000	876,000
Loss on early extinguishment of debt																		0	$ 0	$ 4,911,000	$ 25,071,000	808,000	$ 0	$ 0	$ 0	29,982,000	808,000	$ 8,215,000
Issuance of notes																										0	38,200,000	0
Restricted cash																		12,146,000				22,512,000				12,146,000	22,512,000	$ 27,008,000
Number of swaps terminated | swap			2			1																						7
Notional amount of terminated swaps			$ 120,000,000			$ 50,000,000																						$ 215,000,000
Payment of mortgages payable																										46,000,000	0	65,877,000
Mortgage prepayment penalty																										5,660,000	$ 0	6,361,000
Number of multifamily properties sold | property																											0
Number of mortgage loans paid off | mortgage_loan												2
Accrued interest and unused commitment fee payable																		650,000				1,574,000				650,000	$ 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 | Base rate | Minimum
Debt
Interest rate margin																	0.20%
Unsecured credit facility entered into in September 2014, revolver | Base rate | 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 | 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, 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, delayed-draw term loan
Debt
Maximum borrowing capacity																	$ 75,000,000
Unsecured term facility entered into in May 2015
Debt
Deferred loan costs, accumulated amortization																												705,000
Maximum borrowing capacity																$ 385,000,000
Effective interest rate																3.16%
Repayments of debt																												215,000,000
Write off of deferred loan costs							$ 1,872,000																					1,988,000
Write off of deferred debt issuance cost amortization							1,064,000
Unsecured term facility entered into in May 2015 | Base rate | Minimum
Debt
Interest rate margin																0.60%
Unsecured term facility entered into in May 2015 | Base rate | Maximum
Debt
Interest rate margin																1.25%
Unsecured term facility entered into in May 2015 | LIBOR | Minimum
Debt
Interest rate margin																1.60%
Unsecured term facility entered into in May 2015 | LIBOR | Maximum
Debt
Interest rate margin																2.25%
Mortgages Payable
Debt
Number of mortgage loan agreements | agreement															6
Amount of loan															$ 392,000,000
Outstanding balance																		97,100,000				388,100,000				97,100,000	388,100,000
Mortgage loans with a fixed interest rate of 4.14% per annum
Debt
Outstanding balance																		$ 97,100,000				342,100,000				$ 97,100,000	342,100,000
Revolving Credit Facility
Debt
Maximum borrowing capacity														$ 250,000,000
Effective interest rate																		3.29%								3.29%
Proceeds from issuance of debt							$ 170,000,000
Number of extension options | extension_option														1
Extension term														1 year
Revolving Credit Facility | Minimum
Debt
Unused commitment fee														0.15%
Revolving Credit Facility | Maximum
Debt
Unused commitment fee														0.25%
Revolving Credit Facility | Base rate
Debt
Interest rate margin														0.55%
Revolving Credit Facility | LIBOR
Debt
Interest rate margin														1.55%
Loan Backed Notes
Debt
Issuance of notes								$ 38,200,000
Weighted average life of notes								2 years
Restricted cash																		$ 3,306,000				3,174,000				$ 3,306,000	3,174,000
Outstanding balance																		22,282,000				33,769,000				$ 22,282,000	33,769,000
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%
7083 Hollywood Boulevard | Disposal Group, Held-for-sale, Not Discontinued Operations
Debt
Write off of deferred loan costs										$ 259,000
Write off of deferred debt issuance cost amortization										32,000
Mortgage assumed by buyer										21,700,000
Loss on early extinguishment of debt																												367,000
Transaction costs										$ 140,000
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 loans with a fixed interest rate of 4.14% per annum
Debt
Write off of deferred loan costs				$ 637,000
Write off of deferred debt issuance cost amortization				170,000
Loss on early extinguishment of debt																										$ 19,290,000
Transaction costs				237,000
Defeased amount	$ 39,500,000			205,500,000
Cash outlay required for defeasance				224,086,000
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 deferred debt issuance cost amortization																										65,000
Mortgage assumed by buyer		$ 28,200,000
Loss on early extinguishment of debt																										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 loans with a fixed interest rate of 4.14% per annum
Debt
Write off of deferred loan costs	287,000
Write off of deferred debt issuance cost amortization	82,000
Loss on early extinguishment of debt																										4,911,000
Transaction costs	98,000
Defeased amount	39,500,000
Cash outlay required for defeasance	$ 44,108,000
Number of mortgage loans defeased | mortgage_loan	1
211 Main Street, San Francisco, CA (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Write off of deferred loan costs													$ 165,000
Write off of deferred debt issuance cost amortization													139,000
Loss on early extinguishment of debt																												1,534,000
Payment of mortgages payable													25,331,000
Mortgage prepayment penalty													$ 1,508,000
3636 McKinney Avenue and 3839 McKinney Avenue, Dallas, Texas | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Write off of deferred loan costs												$ 298,000
Write off of deferred debt issuance cost amortization												185,000
Loss on early extinguishment of debt																												2,014,000
Payment of mortgages payable												15,448,000
Mortgage prepayment penalty												$ 1,901,000
4649 Cole Avenue, Dallas, TX (2) | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Write off of deferred loan costs											$ 304,000
Write off of deferred debt issuance cost amortization											191,000
Loss on early extinguishment of debt																												2,925,000
Payment of mortgages payable											23,333,000
Mortgage prepayment penalty											$ 2,812,000
4200 Scotland Street, Houston, TX | Disposal Group, Held-for-sale, Not Discontinued Operations
Debt
Write off of deferred loan costs									$ 264,000
Write off of deferred debt issuance cost amortization									172,000
Mortgage assumed by buyer									$ 28,560,000
Loss on early extinguishment of debt																												$ 92,000
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 deferred debt issuance cost amortization				259,000
Loss on early extinguishment of debt																										5,603,000
Payment of mortgages payable				46,000,000
Mortgage prepayment penalty				$ 5,325,000
Interest rate swaps
Debt
Derivative, notional amount																													$ 385,000,000
Interest rate swaps | Cash flow hedges | Designated as hedging instrument
Debt
Derivative, notional amount																		$ 120,000,000								$ 120,000,000
Interest rate swaps | Cash flow hedges | Designated as hedging instrument | Weighted Average
Debt
All-in rate percentage																		3.11%								3.11%
Revolving Credit Facility | Line of Credit
Debt
Repayments of debt			$ 120,000,000		$ 10,000,000	$ 40,000,000
Outstanding balance																		$ 153,000,000				130,000,000				$ 153,000,000	130,000,000
Amount available for future borrowings																		$ 73,900,000				$ 91,000,000				$ 73,900,000	$ 91,000,000
Multifamily
Debt
Number of multifamily properties sold | property												2