DEBT - Narrative (Details)												1 Months Ended					3 Months Ended	12 Months Ended
	Mar. 01, 2019 USD ($)	Dec. 28, 2018 USD ($) swap	Oct. 30, 2018 USD ($)	May 30, 2018 USD ($)	Dec. 15, 2017 USD ($)	Nov. 29, 2017 USD ($) swap	Sep. 21, 2017 USD ($)	Aug. 03, 2017 USD ($) swap	Jun. 23, 2017 USD ($)	May 30, 2017 USD ($) property mortgage_loan	Mar. 28, 2017 USD ($)	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($) extension_option	Jun. 30, 2016 USD ($) agreement	May 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Mar. 18, 2019 USD ($) swap	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($)	Aug. 13, 2015 USD ($)
Debt
Deferred loan costs, gross												$ 5,994,000						$ 5,994,000	$ 3,843,000
Deferred loan costs, accumulated amortization												876,000						876,000	1,105,000
Restricted cash												22,512,000						22,512,000	27,008,000	$ 32,160,000
Number of swaps terminated | swap		1				4		3
Notional amount of terminated swaps		$ 50,000,000				$ 150,000,000		$ 65,000,000
Proceeds from termination payments, net of fees		684,000																	1,011,000
Payment of mortgages payable																		$ 0	65,877,000	4,354,000
Number of multifamily properties sold | property																		0
Number of mortgage loans paid off | mortgage_loan										2
Issuance of notes																		$ 38,200,000	0	$ 0
Accrued interest and unused commitment fee payable												$ 1,574,000						$ 1,574,000	2,098,000
Payment of derivative fees and accrued interest																			38,000
Unsecured credit facility entered into in September 2014
Debt
Maximum borrowing capacity																$ 850,000,000
Amount outstanding																			0
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						512,000		193,000
Maximum borrowing capacity															$ 385,000,000
Repayments of debt						150,000,000		65,000,000
Write off of deferred loan costs			$ 1,872,000			$ 1,387,000		$ 601,000
Amount of loan																			$ 170,000,000
Interest rate																			2.96%
Fixed interest rate												3.16%						3.16%
Repayments of long-term debt			170,000,000
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
Amount of loan														$ 392,000,000
Number of mortgage loan agreements entered into | agreement														6
Outstanding balance												$ 388,100,000						$ 388,100,000	$ 416,300,000
Mortgage loans with a fixed interest rate of 4.14% per annum
Debt
Outstanding balance												$ 342,100,000						$ 342,100,000	370,300,000
Revolving Credit Facility
Debt
Maximum borrowing capacity													$ 250,000,000
Interest rate												4.07%						4.07%
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
Restricted cash												$ 3,174,000						$ 3,174,000
Outstanding balance												33,769,000						$ 33,769,000	0
Issuance of notes				$ 38,200,000
Weighted average life of notes				2 years
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%
Disposal Group, Held-for-sale, Not Discontinued Operations | Mortgage loans with a fixed interest rate of 4.14% per annum
Debt
Mortgage assumed by buyer												28,200,000
7083 Hollywood Boulevard | Disposal Group, Held-for-sale, Not Discontinued Operations
Debt
Mortgage assumed by buyer							$ 21,700,000
211 Main Street, San Francisco, CA | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
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
Payment of mortgages payable										$ 15,448,000
Mortgage prepayment penalty										$ 1,901,000
4649 Cole Avenue, Dallas, TX | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
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
Mortgage assumed by buyer					$ 28,560,000
Subsequent event
Debt
Number of swaps terminated | swap																	2
Notional amount of terminated swaps																	$ 120,000,000
Proceeds from termination payments, net of fees																	1,302,000
Subsequent event | 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
Defeased amount	$ 205,500,000
Subsequent event | Office Property, San Fransisco California | Disposal Group, Held-for-sale, Not Discontinued Operations
Debt
Mortgage assumed by buyer	28,200,000
Subsequent event | Office Property, Washington, D.C | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Debt
Payment of mortgages payable	46,000,000
Mortgage prepayment penalty	$ 5,326,000
Interest rate swap
Debt
Derivative, notional amount																					$ 385,000,000
Interest rate swap | Cash flow hedges | Designated as hedging instrument
Debt
Derivative, notional amount												120,000,000						$ 120,000,000
Revolving Credit Facility | Line of Credit
Debt
Repayments of debt		$ 40,000,000
Outstanding balance												130,000,000						130,000,000	$ 0
Amount available for future borrowings												$ 91,000,000						$ 91,000,000
Revolving Credit Facility | Subsequent event | Line of Credit
Debt
Repayments of debt																	$ 130,000,000
Multifamily
Debt
Number of multifamily properties sold | property										2									5