DEBT (Details)		9 Months Ended
		Sep. 30, 2018 USD ($) Integer	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Secured and Unsecured Debt [Line Items]
Proceeds from unsecured bank credit facilities		$ 311,641,000	$ 281,342,000
Repayments of Bank Debt		336,789,000	$ 255,988,000
Loans Payable, Noncurrent [Abstract]
Secured Debt		191,292,000		$ 199,512,000
Unsecured Debt		723,300,000		713,061,000
Unsecured bank credit facilities		169,261,000		195,709,000
Total debt		1,083,853,000		1,108,282,000
Secured and unsecured debt [Member]
Loans Payable, Noncurrent [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Remainder of Fiscal Year		2,882,000
Payments of principal over future years [Abstract]
2019		130,570,000
2020		114,096,000
2021		129,563,000
2022		107,769,000
2023 and beyond		432,043,000
Long-term debt, maturities, secured debt and unsecured debt		916,923,000
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		171,191,000		116,339,000
Unsecured bank credit facilities - fixed rate, carrying amount	[1],[2]	0		80,000,000
Unamortized debt issuance costs		(1,930,000)		(630,000)
Unsecured bank credit facilities		169,261,000		195,709,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[2]	725,000,000		715,000,000
Unamortized debt issuance costs		(1,700,000)		(1,939,000)
Unsecured Debt		723,300,000		713,061,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[2]	191,923,000		200,354,000
Unamortized debt issuance costs		(631,000)		(842,000)
Secured Debt		191,292,000		$ 199,512,000
Nine bank group unsecured revolving credit facility [Member] | Debt with effectively fixed interest rate [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End				2.02%
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities				$ 80,000,000
Nine bank group unsecured revolving credit facility [Member] | Former facility - $300 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 300,000,000
Debt instrument, basis spread about LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Debt Instrument, Maturity Date, Description		Jul. 30, 2019
Nine bank group unsecured revolving credit facility [Member] | Bank credit facility obtained in 2018 - $350 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		3.218%
Letters of Credit Outstanding, Amount		$ 674,000
Line of Credit Facility, Current Borrowing Capacity		$ 350,000,000
Number of banks included in the unsecured revolving credit facility | Integer		9
Debt instrument, basis spread about LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Debt Instrument, Maturity Date, Description		Jul. 30, 2022
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 155,000,000
Payments of principal over future years [Abstract]
Extension option on credit facility		two six-month extensions
Expansion option on credit facility		$ 150,000,000
Pnc Na Unsecured revolving credit facility [Member] | Former credit facility - $35 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 35,000,000
Debt instrument, basis spread about LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Pnc Na Unsecured revolving credit facility [Member] | Credit facility obtained in 2018 - $45 million [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End		3.261%
Line of Credit Facility, Current Borrowing Capacity		$ 45,000,000
Debt instrument, basis spread about LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Debt Instrument, Maturity Date, Description		Jul. 30, 2022
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 16,191,000
Payments of principal over future years [Abstract]
Extension option on credit facility		two six-month extensions

[1] The Company had designated an interest rate swap to an $80 million unsecured bank credit facility draw that effectively fixed the interest rate on the $80 million draw to 2.020% through the interest rate swap's maturity date. This swap matured on August 15, 2018, and the $80 million draw has reverted to the variable interest rate associated with the Company's unsecured bank credit facilities.

[2] These loans have a fixed interest rate or an effectively fixed interest rate due to interest rate swaps.