DEBT (Details)		3 Months Ended	6 Months Ended	12 Months Ended
		Jun. 30, 2019 USD ($) Integer	Jun. 30, 2019 USD ($) Integer	Dec. 31, 2018 USD ($)
Secured and Unsecured Debt [Line Items]
Document Period End Date			Jun. 30, 2019
Loans Payable, Noncurrent [Abstract]
Secured Debt		$ 137,493,000	$ 137,493,000	$ 188,461,000
Unsecured Debt		803,534,000	803,534,000	723,400,000
Unsecured bank credit facilities		194,327,000	194,327,000	193,926,000
Total debt		1,135,354,000	1,135,354,000	1,105,787,000
Secured and unsecured debt [Member]
Payments of principal over future years [Abstract]
Remainder of 2019		79,469,000	79,469,000
2020		114,096,000	114,096,000
2021		129,562,000	129,562,000
2022		107,770,000	107,770,000
2023		115,119,000	115,119,000
2024 and beyond		396,925,000	396,925,000
Long-term debt, maturities, secured debt and unsecured debt		942,941,000	942,941,000
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		195,880,000	195,880,000	195,730,000
Unamortized debt issuance costs		(1,553,000)	(1,553,000)	(1,804,000)
Unsecured bank credit facilities		194,327,000	194,327,000	193,926,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[1]	805,000,000	805,000,000	725,000,000
Unamortized debt issuance costs		(1,466,000)	(1,466,000)	(1,600,000)
Unsecured Debt		803,534,000	803,534,000	723,400,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[1]	137,941,000	137,941,000	189,038,000
Unamortized debt issuance costs		(448,000)	(448,000)	(577,000)
Secured Debt		$ 137,493,000	$ 137,493,000	188,461,000
$80 million senior unsecured private placement notes (2019) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		10 years
Debt Instrument, Interest Rate, Effective Percentage		4.27%	4.27%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 80,000,000	$ 80,000,000
Pnc Na Unsecured revolving credit facility [Member] | Former credit facility - $35 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity				$ 35,000,000
Debt instrument, basis spread above LIBOR variable rate				100
Line of credit, facility fee (in basis points)				20
Debt Instrument, Maturity Date, Description				July 30, 2019
Pnc Na Unsecured revolving credit facility [Member] | Credit facility obtained in 2018 - $45 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		3.398%	3.398%
Line of Credit Facility, Current Borrowing Capacity		$ 45,000,000	$ 45,000,000
Debt instrument, basis spread above LIBOR variable rate			100
Line of credit, facility fee (in basis points)			20
Debt Instrument, Maturity Date, Description			Jul. 30, 2022
Extension option on credit facility			two six-month extensions
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 30,880,000	$ 30,880,000
Nine bank group unsecured revolving credit facility [Member] | Former facility - $300 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity				$ 300,000,000
Debt instrument, basis spread above LIBOR variable rate				100
Line of credit, facility fee (in basis points)				20
Debt Instrument, Maturity Date, Description				Jul. 30, 2019
Nine bank group unsecured revolving credit facility [Member] | Bank credit facility obtained in 2018 - $350 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		3.398%	3.398%
Letters of Credit Outstanding, Amount		$ 674,000	$ 674,000
Line of Credit Facility, Current Borrowing Capacity		$ 350,000,000	$ 350,000,000
Number of banks included in the unsecured revolving credit facility | Integer		9	9
Debt instrument, basis spread above LIBOR variable rate			100
Line of credit, facility fee (in basis points)			20
Debt Instrument, Maturity Date, Description			Jul. 30, 2022
Extension option on credit facility			two six-month extensions
Expansion option on credit facility			$ 150,000,000
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 165,000,000	$ 165,000,000
Nine bank group unsecured revolving credit facility [Member] | $80 million interest rate swap [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End				2.02%
Unsecured bank credit facilities - fixed rate, carrying amount				$ 80,000,000
Debt Instrument, Maturity Date, Description				Aug. 15, 2018

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