DEBT (Details)		9 Months Ended
		Sep. 30, 2020 USD ($) Integer	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Secured and Unsecured Debt [Line Items]
Repayments of Unsecured Debt		$ (30,000,000)	$ (75,000,000)
Loans Payable, Noncurrent [Abstract]
Secured Debt		126,515,000		$ 133,093,000
Unsecured Debt		1,008,012,000		938,115,000
Unsecured bank credit facilities		76,540,000		111,394,000
Total debt		$ 1,211,067,000		1,182,602,000
$100 million senior unsecured private placement notes (2020) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		7 years
Debt Instrument, Interest Rate, Effective Percentage		2.39%
Debt Instrument, Basis Spread on Variable Rate		1.45%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 100,000,000
Secured and unsecured debt [Member]
Payments of principal over future years [Abstract]
Remainder of 2020		77,299,000
2021		129,562,000
2022		107,770,000
2023		115,119,000
2024		120,122,000
2025 and beyond		586,802,000
Long-term debt, maturities, secured debt and unsecured debt		$ 1,136,674,000
$175 million senior unsecured private placement notes (2020) [Member]
Secured and Unsecured Debt [Line Items]
Fixed interest rate		2.65%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 175,000,000
$100 million senior unsecured private placement notes (3Q 2020) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		10 years
Fixed interest rate		2.61%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 100,000,000
$75 million senior unsecured private placement notes (3Q 2020) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		12 years
Fixed interest rate		2.71%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 75,000,000
$100 million senior unsecured private placement notes (2013) [Member]
Secured and Unsecured Debt [Line Items]
Repayments of Unsecured Debt		$ 30,000,000
Fixed interest rate		3.80%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 100,000,000
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		77,474,000		112,710,000
Unamortized debt issuance costs		(934,000)		(1,316,000)
Unsecured bank credit facilities		76,540,000		111,394,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[1]	1,010,000,000		940,000,000
Unamortized debt issuance costs		(1,988,000)		(1,885,000)
Unsecured Debt		1,008,012,000		938,115,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt-fixed rate, carrying amount	[1]	126,674,000		133,422,000
Unamortized debt issuance costs		(159,000)		(329,000)
Secured Debt		$ 126,515,000		$ 133,093,000
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		1.148%
Line of Credit Facility, Current Borrowing Capacity		$ 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		July 30, 2022
Extension option on credit facility		two six-month extensions
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 12,474,000
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		1.146%
Letters of Credit Outstanding, Amount		$ 674,000
Line of Credit Facility, Current Borrowing Capacity		$ 350,000,000
Number of banks included in the unsecured revolving credit facility | Integer		9
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, 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		$ 65,000,000

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