UNSECURED BANK CREDIT FACILITIES (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) Integer	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Unsecured bank credit facilities, net of debt issuance costs	$ 207,066,000	$ 124,194,000
Average bank borrowings	$ 95,629,000	$ 87,095,000	$ 172,175,000
Weighted average interest rates (in hundredths)	1.01%	1.86%	3.34%
Amortization of facility fees	$ 751,000	$ 790,000	$ 790,000
Amortization of debt issuance costs	606,000	$ 561,000	$ 556,000
$425 million credit facility - obtained in 2021
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 425,000,000
Debt instrument, spread above LIBOR variable rate (in basis points)	77.5
Line of credit, facility fee (in basis points)	15
Credit facility, extension option	two six-month extensions
Banks included in the unsecured revolving credit facility group | Integer	9
Line of Credit Facility, Interest Rate at Period End	0.875%
Unsecured bank credit facilities - variable rate, carrying amount	$ 183,000,000
Letters of Credit Outstanding, Amount	674,000
Expansion option on credit facility	$ 325,000,000
Debt Instrument, Maturity Date	Jul. 30, 2025
$50 million credit facility - obtained in 2021
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 50,000,000
Debt instrument, spread above LIBOR variable rate (in basis points)	77.5
Line of credit, facility fee (in basis points)	15
Credit facility, extension option	two six-month extensions
Line of Credit Facility, Interest Rate at Period End	0.876%
Unsecured bank credit facilities - variable rate, carrying amount	$ 26,210,000
Debt Instrument, Maturity Date	Jul. 30, 2025
Credit facilities - obtained in 2021
Line of Credit Facility [Line Items]
Favorable pricing level - leverage ratio maximum	32.50%
Debt Instrument, Credit Rating	Baa2
Sustainability-linked pricing component, interest margin reduction (in basis points)	1
Former bank credit facility obtained in 2018 - $350 million | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Former Borrowing Capacity	$ 350,000,000
Former credit facility obtained in 2018 - $45 million [Member] | Pnc Na Unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Former Borrowing Capacity	$ 45,000,000
Former credit facilities obtained in 2018
Line of Credit Facility [Line Items]
Debt instrument, spread above LIBOR variable rate (in basis points)	100
Line of credit, facility fee (in basis points)	20
Debt Instrument, Maturity Date, Description	July 30, 2022