UNSECURED BANK CREDIT FACILITIES (Details)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Line of Credit Facility [Line Items]
Weighted average interest rates (in hundredths)	1.85%	1.92%
Weighted average interest rate (including amortization of loan costs)	2.30%	2.35%
Notes payable to banks	$ 150,836,000	$ 99,401,000
Average bank borrowings	109,777,000	96,162,000
Amortization of bank loan costs	493,000	$ 413,000	$ 410,000
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, Interest Rate at Period End	1.43%
Notes payable to banks	$ 13,836,000
Extension option on credit facility	1 year
Facility fee (in basis points)	20
Former nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 225,000,000
Current bank credit facilities [Member]
Line of Credit Facility [Line Items]
Debt instrument, spread above LIBOR variable rate (in basis points)	100
Facility fee (in basis points)	20
Former credit facilities [Member]
Line of Credit Facility [Line Items]
Debt instrument, spread above LIBOR variable rate (in basis points)	117.5
Line of credit, facility fee (in basis points)	22.5
Former Pnc na revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 25,000,000
Nine bank group unsecured revolving credit facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 300,000,000
Banks included in the unsecured revolving credit facility group	9
Line of Credit Facility, Interest Rate at Period End	1.394%
Notes payable to banks	$ 137,000,000
Extension option on credit facility	1 year
Expansion option on credit facility	$ 150,000,000