DEBT (Details) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Secured and Unsecured Debt [Line Items]
Repayments of Bank Debt	$ 388,569,000	$ 229,508,000
Proceeds from Issuance of Unsecured Debt	105,000,000	75,000,000
Repayments on Unsecured Debt	(80,000,000)	0
Unsecured bank credit facilities, change during period	56,037,000
Proceeds from unsecured bank credit facilities	444,314,000	$ 273,253,000
Loans Payable, Noncurrent [Abstract]
Secured Debt	260,910,000		$ 350,285,000
Unsecured Debt	553,035,000		528,210,000
Unsecured bank credit facilities	205,451,000		149,414,000
Total debt	1,019,396,000		1,027,909,000
Secured Debt [Member]
Secured and Unsecured Debt [Line Items]
Secured debt, change during period	$ (89,375,000)
Number of secured loans repaid during period	2
Secured debt, amount repaid during period	$ 75,737,000
Secured debt, principal payments during period	13,558,000
Secured and unsecured debt [Member]
Payments of principal over future years [Abstract]
Remainder of 2016	3,485,000
2017	58,239,000
2018	61,316,000
2019	130,569,000
2020	114,097,000
2021 and beyond	449,375,000
Long-term debt, maturities, secured debt and unsecured debt	817,081,000
Unsecured Debt [Member]
Secured and Unsecured Debt [Line Items]
Unsecured debt, change during period	24,825,000
Sixty-five million term loan (2016) [Member]
Secured and Unsecured Debt [Line Items]
Unsecured debt, notional amount	$ 65,000,000
Unsecured debt, term (in years)	7
Interest Rate (in hundredths)	2.863%
Long-term debt, margin over LIBOR rate	1.65%
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	$ 262,081,000		351,401,000
Unamortized debt issuance costs	(1,171,000)		(1,116,000)
Secured Debt	260,910,000		350,285,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	555,000,000		530,000,000
Unamortized debt issuance costs	(1,965,000)		(1,790,000)
Unsecured Debt	553,035,000		528,210,000
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount	126,581,000		150,836,000
Unsecured bank credit facilities - fixed rate, carrying amount	80,000,000		0
Unamortized debt issuance costs	(1,130,000)		(1,422,000)
Unsecured bank credit facilities	205,451,000		$ 149,414,000
$40 million term loan (2016) [Member]
Secured and Unsecured Debt [Line Items]
Unsecured debt, notional amount	$ 40,000,000
Unsecured debt, term (in years)	5
Interest Rate (in hundredths)	2.335%
Long-term debt, margin over LIBOR rate	1.10%
$80 million term loan (repaid in 2016) [Member]
Secured and Unsecured Debt [Line Items]
Unsecured debt, notional amount	$ 80,000,000
Interest Rate (in hundredths)	2.77%
Nine bank group unsecured revolving credit facility [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 300,000,000
Nine bank group unsecured revolving credit facility [Member] | $80 million draw on unsecured bank credit facilities - fixed rate due to interest rate swap [Member]
Secured and Unsecured Debt [Line Items]
Interest Rate (in hundredths)	2.02%
Proceeds from unsecured bank credit facilities	$ 80,000,000