DEBT (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Secured and Unsecured Debt [Line Items]
Proceeds from unsecured bank credit facilities		$ 91,387	$ 84,734
Repayments of Bank Debt		85,634	$ 76,518
Loans Payable, Noncurrent [Abstract]
Secured Debt		196,809		$ 199,512
Unsecured Debt		713,122		713,061
Unsecured bank credit facilities		201,561		195,709
Total debt		1,111,492		1,108,282
Secured and unsecured debt [Member]
Loans Payable, Noncurrent [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Remainder of Fiscal Year		58,540
Payments of principal over future years [Abstract]
2019		130,569
2020		114,096
2021		129,563
2022		107,769
2023 and beyond		372,043
Long-term debt, maturities, secured debt and unsecured debt		912,580
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		122,092		116,339
Unsecured bank credit facilities - fixed rate, carrying amount	[1]	80,000		80,000
Unamortized debt issuance costs		531		630
Unsecured bank credit facilities		201,561		195,709
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[1]	715,000		715,000
Unamortized debt issuance costs		1,878		1,939
Unsecured Debt		713,122		713,061
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[1]	197,580		200,354
Unamortized debt issuance costs		771		842
Secured Debt		$ 196,809		$ 199,512

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