DEBT (Details)		9 Months Ended
		Sep. 30, 2021 USD ($) basisPoints years Integer	Dec. 31, 2020 USD ($)
Secured and Unsecured Debt [Line Items]
Moody's Credit Rating		Baa2
Unsecured bank credit facilities		$ 58,719,000	$ 124,194,000
Secured Debt		35,466,000	78,993,000
Unsecured Debt		1,242,430,000	1,107,708,000
Total debt		1,336,615,000	1,310,895,000
Secured and unsecured debt, net of debt issuance costs [Member]
Payments of principal over future years [Abstract]
Remainder of 2021		678,000
2022		107,770,000
2023		115,119,000
2024		120,122,000
2025		145,128,000
2026 and beyond		791,673,000
Long-term debt, maturities, secured debt and unsecured debt		$ 1,280,490,000
$50 million senior unsecured term loan (2021)
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		4 years
Debt Instrument, Interest Rate, Effective Percentage		1.55%
Debt Instrument, Basis Spread on Variable Rate		1.00%
Unsecured debt, carrying amount		$ 50,000,000
Mortgage loan, repaid during period
Secured and Unsecured Debt [Line Items]
Secured debt, carrying amount repaid		$ 40,800,000
Debt Instrument, Maturity Date, Description		June 5, 2021
Fixed interest rate		4.75%
$125 million senior unsecured private placement notes (2021)
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		10 years
Fixed interest rate		2.74%
Unsecured debt, carrying amount		$ 125,000,000
Term loan, repaid during period
Secured and Unsecured Debt [Line Items]
Fixed interest rate		2.34%
Unsecured debt, carrying amount repaid		$ 40,000,000
$100 million senior unsecured private placement note (refinanced in 2021)
Secured and Unsecured Debt [Line Items]
Basis point reduction in credit spread | basisPoints		65
Sustainability performance linked basis point reduction (in basis points) | years		1
Debt term, years remaining		5 years
Debt instrument, basis spread above LIBOR variable rate | basisPoints		85
Fixed interest rate		2.10%
Unsecured debt, carrying amount		$ 100,000,000
Unsecured bank credit facilities
Secured and Unsecured Debt [Line Items]
Unsecured bank credit facilities - variable rate, carrying amount		61,016,000	125,000,000
Unsecured bank credit facilities		58,719,000	124,194,000
Unamortized debt issuance costs		(2,297,000)	(806,000)
Unsecured Debt [Member]
Secured and Unsecured Debt [Line Items]
Unamortized debt issuance costs		(2,570,000)	(2,292,000)
Unsecured debt, carrying amount	[1]	1,245,000,000	1,110,000,000
Unsecured Debt		1,242,430,000	1,107,708,000
Secured Debt [Member]
Secured and Unsecured Debt [Line Items]
Secured debt-fixed rate, carrying amount	[1]	35,490,000	79,096,000
Secured Debt		35,466,000	78,993,000
Unamortized debt issuance costs		(24,000)	$ (103,000)
Nine bank group unsecured revolving credit facility [Member] | Former credit facility obtained in 2018 - $350 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Borrowing Capacity		$ 350,000,000
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
Nine bank group unsecured revolving credit facility [Member] | Bank credit facility obtained in 2021 - $425 million
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		0.861%
Letters of Credit Outstanding, Amount		$ 674,000
Line of Credit Facility, Borrowing Capacity		$ 425,000,000
Number of banks included in the unsecured revolving credit facility | Integer		9
Debt instrument, basis spread above LIBOR variable rate		77.5
Line of credit, facility fee (in basis points)		15
Debt Instrument, Maturity Date, Description		July 30, 2025
Extension option on credit facility		two six-month extensions
Line of credit facility, accordion		$ 325,000,000
Unsecured bank credit facilities - variable rate, carrying amount		$ 45,000,000
Nine bank group unsecured revolving credit facility [Member] | Bank credit facilities obtained in 2021 - $425 and $50 million
Secured and Unsecured Debt [Line Items]
Sustainability performance linked basis point reduction (in basis points)		1
Initial pricing basis for credit facilities		BBB+/Baa1
Line of credit facility covenant terms, consolidated leverage ratio		32.50%
Pnc Na Unsecured revolving credit facility [Member] | Former credit facility obtained in 2018 - $45 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, 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
Pnc Na Unsecured revolving credit facility [Member] | Bank credit facility obtained in 2021 - $50 million
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		0.855%
Line of Credit Facility, Borrowing Capacity		$ 50,000,000
Debt instrument, basis spread above LIBOR variable rate		77.5
Line of credit, facility fee (in basis points)		15
Debt Instrument, Maturity Date, Description		July 30, 2025
Extension option on credit facility		two six-month extensions
Unsecured bank credit facilities - variable rate, carrying amount		$ 16,016,000

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