REAL ESTATE PROPERTIES (Details)		3 Months Ended			9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) Integer	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items]
Percentage of occupancy to qualify as value-add property					75.00%
Percentage of acquisition price to be spent on capital items to qualify as value-add property					20.00%
Number of properties impacted by involuntary conversion | Integer			2
Amortization expense for lease intangibles		$ 1,157,000		$ 1,101,000	$ 3,202,000	$ 3,455,000
Depreciation Expense During the Period		18,960,000		17,323,000	55,785,000	51,070,000
Real Estate Properties
Land		376,658,000			376,658,000		$ 345,424,000
Building and building improvements		1,703,237,000			1,703,237,000		1,587,130,000
Tenant and other improvements		431,751,000			431,751,000		404,180,000
Development and value-add properties	[1]	244,761,000			244,761,000		242,014,000
Real estate, development and value-add properties		2,756,407,000			2,756,407,000		2,578,748,000
Less accumulated depreciation		(796,037,000)			(796,037,000)		(749,601,000)
Real estate, net		1,960,370,000			1,960,370,000		1,829,147,000
Rental Income Increase Decrease For Amortization Of Acquired In Place Lease Intangibles		(217,000)		$ (129,000)	(477,000)	(406,000)
Gain on casualties and involuntary conversion			$ 1,150,000		$ 1,150,000	$ 0
Building [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life					40 years
Minimum [Member] | Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life					3 years
Maximum [Member] | Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life					15 years
2018 Acquisitions [Member]
Real Estate Properties
Total cost of properties purchased					$ 66,868,000
Amount of total cost allocated to land		22,461,000			22,461,000
2017 Acquisitions [Member]
Real Estate Properties
Total cost of properties purchased							65,243,000
Amount of total cost allocated to land							11,281,000
Below market lease [Member] | 2018 Acquisitions [Member]
Real Estate Properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		1,618,000			1,618,000
Below market lease [Member] | 2017 Acquisitions [Member]
Real Estate Properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles							385,000
Above Market Leases [Member] | 2018 Acquisitions [Member]
Real Estate Properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		21,000			21,000
Above Market Leases [Member] | 2017 Acquisitions [Member]
Real Estate Properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles							115,000
Industrial Development [Member] | 2018 Acquisitions [Member]
Real Estate Properties
Amount of total cost allocated to real estate properties		$ 13,934,000			$ 13,934,000
Industrial Development [Member] | 2017 Acquisitions [Member]
Real Estate Properties
Amount of total cost allocated to real estate properties							$ 10,312,000

[1] Value-add properties are defined as properties that are either acquired but not stabilized or can be converted to a higher and better use. Acquired properties meeting either of the following two conditions are considered value-add properties: (1) Less than 75% occupied as of the acquisition date (or will be less than 75% occupied within one year of acquisition date based on near term lease roll), or (2) 20% or greater of the acquisition cost will be spent to redevelop the property.