UNSECURED BANK CREDIT FACILITIES (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Integer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Average bank borrowings	$ 114,751,000	$ 106,352,000	$ 109,777,000
Weighted average interest rates (in hundredths)	2.07%	1.49%	1.29%
Amortization of facility fees	$ 670,000	$ 670,000	$ 608,000
Amortization of bank loan costs	451,000	450,000	$ 493,000
Unsecured bank credit facilities	195,709,000	$ 190,990,000
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
Line of Credit Facility, Interest Rate at Period End	2.564%
Unsecured bank credit facilities	$ 6,339,000
Extension option on credit facility	1 year
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
Banks included in the unsecured revolving credit facility group | Integer	9
Letters of Credit Outstanding, Amount	$ 674,000
Extension option on credit facility	1 year
Expansion option on credit facility	$ 150,000,000
Debt with effectively fixed interest rate [Member] | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.02%
Unsecured bank credit facilities	$ 80,000,000
Debt with variable interest rate [Member] | Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.528%
Unsecured bank credit facilities	$ 110,000,000