UNSECURED AND SECURED DEBT (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Secured and Unsecured Debt [Line Items]
Carrying Amount of Securing Real Estate		$ 239,574,000
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		715,000,000	$ 655,000,000
Secured debt, carrying amount		200,354,000	258,594,000
Unsecured bank credit facilities		195,709,000	190,990,000
Unsecured debt		713,061,000	652,838,000
Secured debt		199,512,000	257,505,000
Total debt		1,108,282,000	1,101,333,000
Secured and unsecured debt [Member]
Payments of principal over the next five years [Abstract]
2018		61,314,000
2019		130,569,000
2020		114,096,000
2021		129,563,000
2022		$ 107,769,000
$50 Million Unsecured Term Loan [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.91%
Debt Instrument, Maturity Date		Dec. 21, 2018
Unsecured debt, carrying amount		$ 50,000,000	50,000,000
Seventy-Five Million Unsecured Term Loan (closed in 2014) [Member]
Secured and Unsecured Debt [Line Items]
Debt instrument, spread above LIBOR variable rate	[1]	1.15%
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)	[1]	2.846%
Debt Instrument, Maturity Date	[1]	Jul. 31, 2019
Unsecured debt, carrying amount	[1]	$ 75,000,000	75,000,000
$75 Million Unsecured Term Loan [Member]
Secured and Unsecured Debt [Line Items]
Debt instrument, spread above LIBOR variable rate	[1]	1.10%
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)	[1]	3.452%
Debt Instrument, Maturity Date	[1]	Dec. 20, 2020
Unsecured debt, carrying amount	[1]	$ 75,000,000	75,000,000
$40 million term loan (2016) [Member]
Secured and Unsecured Debt [Line Items]
Debt instrument, spread above LIBOR variable rate	[1]	1.10%
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)	[1]	2.335%
Debt Instrument, Maturity Date	[1]	Jul. 30, 2021
Unsecured debt, carrying amount	[1]	$ 40,000,000	40,000,000
Seventy-Five Million Unsecured Term Loan executed in 2015 [Member]
Secured and Unsecured Debt [Line Items]
Debt instrument, spread above LIBOR variable rate	[1]	1.40%
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)	[1]	3.031%
Debt Instrument, Maturity Date	[1]	Feb. 28, 2022
Unsecured debt, carrying amount	[1]	$ 75,000,000	75,000,000
Sixty-five million term loan (2016) [Member]
Secured and Unsecured Debt [Line Items]
Debt instrument, spread above LIBOR variable rate	[1]	1.65%
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)	[1]	2.863%
Debt Instrument, Maturity Date	[1]	Apr. 01, 2023
Unsecured debt, carrying amount	[1]	$ 65,000,000	65,000,000
Thirty million senior unsecured notes [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.80%
Debt Instrument, Maturity Date		Aug. 28, 2020
Unsecured debt, carrying amount		$ 30,000,000	30,000,000
Fifty million senior unsecured notes [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.80%
Debt Instrument, Maturity Date		Aug. 28, 2023
Unsecured debt, carrying amount		$ 50,000,000	50,000,000
Twenty million senior unsecured notes [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.80%
Debt Instrument, Maturity Date		Aug. 28, 2025
Unsecured debt, carrying amount		$ 20,000,000	20,000,000
Sixty million senior unsecured notes (executed in 2017) [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.46%
Debt Instrument, Maturity Date		Dec. 13, 2024
Unsecured debt, carrying amount		$ 60,000,000	0
Sixty million senior unsecured notes (executed in 2016) [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.48%
Debt Instrument, Maturity Date		Dec. 15, 2024
Unsecured debt, carrying amount		$ 60,000,000	60,000,000
Forty million senior unsecured notes (executed in 2016) [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.75%
Debt Instrument, Maturity Date		Dec. 15, 2026
Unsecured debt, carrying amount		$ 40,000,000	40,000,000
Twenty-five million senior unsecured notes (executed in 2015) [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.97%
Debt Instrument, Maturity Date		Oct. 01, 2025
Unsecured debt, carrying amount		$ 25,000,000	25,000,000
Fifty million senior unsecured notes (executed in 2015) [Member]
Loans Payable, Noncurrent [Abstract]
Effective interest rate (in hundredths)		3.99%
Debt Instrument, Maturity Date		Oct. 07, 2025
Unsecured debt, carrying amount		$ 50,000,000	50,000,000
Arion 16, Broadway VI, Chino, East University I and II, Northpark, Santan 10 II, 55th Avenue and World Houston 1 and 2, 21 and 23 [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		5.57%
Monthly P & I Payment		$ 518,885
Maturity Date		Repaid
Carrying Amount of Securing Real Estate		$ 0
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 0	47,496,000
Dominguez, Industry I and III, Kingsview, Shaw, Walnut and Washington [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		7.50%
Monthly P & I Payment		$ 539,747
Maturity Date		5/5/2019
Carrying Amount of Securing Real Estate		$ 45,886,000
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 49,580,000	52,231,000
Blue Heron Distribution Center II [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		5.39%
Monthly P & I Payment		$ 16,176
Maturity Date		2/29/2020
Carrying Amount of Securing Real Estate		$ 4,425,000
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 409,000	576,000
40th Avenue, Beltway Crossing V, Centennial Park, Executive Airport, Interchange Park I, Ocean View, Wetmore 5-8 and World Houston 26, 28, 29 and 30 [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		4.39%
Monthly P & I Payment		$ 463,778
Maturity Date		1/5/2021
Carrying Amount of Securing Real Estate		$ 67,796,000
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 55,317,000	58,380,000
Colorado Crossing, Interstate I-III, Rojas, Steele Creek 1 & 2, Venture and World Houston 3-9
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		4.75%
Monthly P & I Payment		$ 420,045
Maturity Date		6/5/2021
Carrying Amount of Securing Real Estate		$ 56,000,000
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 50,161,000	52,752,000
Arion 18, Beltway VI & VII, Commerce Park II & III, Concord Distribution Center, Interstate Distribution Center V, VI & VII, Lakeview Business Center, Ridge Creek Distribution Center II, Southridge IV & V and World Houston 32 [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		4.09%
Monthly P & I Payment		$ 329,796
Maturity Date		1/5/2022
Carrying Amount of Securing Real Estate		$ 56,440,000
Loans Payable, Noncurrent [Abstract]
Secured debt		$ 42,315,000	44,493,000
Ramona Distribution Center [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)		3.85%
Monthly P & I Payment		$ 16,287
Maturity Date		11/30/2026
Carrying Amount of Securing Real Estate		$ 9,027,000
Loans Payable, Noncurrent [Abstract]
Secured debt		2,572,000	2,666,000
Notes Payable to Banks [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		116,339,000	112,020,000
Unsecured bank credit facilities - fixed rate, carrying amount	[2]	80,000,000	80,000,000
Unamortized debt issuance costs		(630,000)	(1,030,000)
Unsecured bank credit facilities		195,709,000	190,990,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[2]	715,000,000	655,000,000
Unamortized debt issuance costs		(1,939,000)	(2,162,000)
Unsecured debt		713,061,000	652,838,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[2]	200,354,000	258,594,000
Unamortized debt issuance costs		(842,000)	(1,089,000)
Secured debt		$ 199,512,000	$ 257,505,000

[1] The interest rates on these unsecured term loans are comprised of LIBOR plus a margin which is subject to a pricing grid for changes in the Company's coverage ratings. The Company entered into interest rate swap agreements (further described in Note 13) to convert the loans' LIBOR rates to effectively fixed interest rates. The interest rates in the table above are the effectively fixed interest rates for the loans, including the effects of the interest rate swaps, as of December 31, 2017.

[2] These loans have a fixed interest rate or an effectively fixed interest rate due to interest rate swaps.