DEBT (Details)		3 Months Ended		9 Months Ended	12 Months Ended
		Sep. 30, 2019 USD ($) Integer	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) Integer	Dec. 31, 2018 USD ($)
Loans Payable, Noncurrent [Abstract]
Secured Debt		$ 135,330,000		$ 135,330,000	$ 188,461,000
Unsecured Debt		838,581,000		838,581,000	723,400,000
Unsecured bank credit facilities		138,579,000		138,579,000	193,926,000
Total debt		1,112,490,000		1,112,490,000	1,105,787,000
Secured and unsecured debt [Member]
Payments of principal over future years [Abstract]
Remainder of 2019		2,248,000		2,248,000
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		506,924,000		506,924,000
Long-term debt, maturities, secured debt and unsecured debt		$ 975,719,000		$ 975,719,000
$35 million senior unsecured private placement notes (2019) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		12 years
Debt Instrument, Interest Rate, Effective Percentage		3.54%		3.54%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 35,000,000		$ 35,000,000
$75 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		3.47%		3.47%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 75,000,000		$ 75,000,000
$75 million unsecured term loan (repaid in 2019) [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 75,000,000		$ 75,000,000
Payments of principal over future years [Abstract]
Fixed interest rate		2.846%		2.846%
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 140,006,000		$ 140,006,000	195,730,000
Unamortized debt issuance costs		(1,427,000)		(1,427,000)	(1,804,000)
Unsecured bank credit facilities		138,579,000		138,579,000	193,926,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[1]	840,000,000		840,000,000	725,000,000
Unamortized debt issuance costs		(1,419,000)		(1,419,000)	(1,600,000)
Unsecured Debt		838,581,000		838,581,000	723,400,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[1]	135,719,000		135,719,000	189,038,000
Unamortized debt issuance costs		(389,000)		(389,000)	(577,000)
Secured Debt		$ 135,330,000		$ 135,330,000	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%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount			$ 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		3.016%		3.016%
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		$ 30,006,000		$ 30,006,000
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		3.048%		3.048%
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		$ 110,000,000		$ 110,000,000
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.