UNSECURED BANK CREDIT FACILITIES (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) Integer	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Unsecured bank credit facilities	$ 111,394,000	$ 193,926,000
Average bank borrowings	$ 172,175,000	$ 141,223,000	$ 114,751,000
Weighted average interest rates (in hundredths)	3.34%	2.64%	2.07%
Amortization of facility fees	$ 790,000	$ 736,000	$ 670,000
Amortization of bank loan costs	556,000	508,000	$ 451,000
Former credit facility - $35 million [Member] | Pnc Na Unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 35,000,000
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		Jul. 30, 2019
Former facility - $300 million [Member] | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 300,000,000
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		Jul. 30, 2019
Bank credit facility obtained in 2018 - $350 million [Member] | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 350,000,000
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	Jul. 30, 2022
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	2.776%
Unsecured bank credit facilities - variable rate, carrying amount	$ 105,000,000
Letters of Credit Outstanding, Amount	674,000
Expansion option on credit facility	150,000,000
$80 million interest rate swap [Member] | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Maturity Date, Description		Aug. 15, 2018
Line of Credit Facility, Interest Rate at Period End		2.02%
Unsecured bank credit facilities		$ 80,000,000
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, Current Borrowing Capacity	$ 45,000,000
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	Jul. 30, 2022
Credit facility, extension option	two six-month extensions
Line of Credit Facility, Interest Rate at Period End	2.763%
Unsecured bank credit facilities - variable rate, carrying amount	$ 7,710,000