DEBT (Details)		3 Months Ended		12 Months Ended
		Mar. 31, 2019 USD ($) Integer	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Secured and Unsecured Debt [Line Items]
Proceeds from unsecured bank credit facilities		$ 144,635,000	$ 91,387,000
Repayments of Bank Debt		$ 207,497,000	$ 85,634,000
Document Period End Date		Mar. 31, 2019
Loans Payable, Noncurrent [Abstract]
Secured Debt		$ 185,606,000		$ 188,461,000
Unsecured Debt		803,397,000		723,400,000
Unsecured bank credit facilities		131,189,000		193,926,000
Total debt		1,120,192,000		1,105,787,000
Secured and unsecured debt [Member]
Payments of principal over future years [Abstract]
Remainder of 2019		127,647,000
2020		114,096,000
2021		129,562,000
2022		107,769,000
2023		115,119,000
2024 and beyond		396,923,000
Long-term debt, maturities, secured debt and unsecured debt		991,116,000
Unsecured bank credit facilities
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		132,868,000		195,730,000
Unamortized debt issuance costs		(1,679,000)		(1,804,000)
Unsecured bank credit facilities		131,189,000		193,926,000
Unsecured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount	[1]	805,000,000		725,000,000
Unamortized debt issuance costs		(1,603,000)		(1,600,000)
Unsecured Debt		803,397,000		723,400,000
Secured Debt [Member]
Loans Payable, Noncurrent [Abstract]
Secured debt, carrying amount	[1]	186,116,000		189,038,000
Unamortized debt issuance costs		(510,000)		(577,000)
Secured Debt		$ 185,606,000		188,461,000
$80 million senior unsecured private placement notes (2019) [Member]
Secured and Unsecured Debt [Line Items]
Debt Instrument, Term		10 years
Debt Instrument, Interest Rate, Effective Percentage		4.27%
Loans Payable, Noncurrent [Abstract]
Unsecured debt, carrying amount		$ 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 above 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		3.491%
Letters of Credit Outstanding, Amount		$ 704,740
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 above LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Debt Instrument, Maturity Date, Description		Jul. 30, 2022
Extension option on credit facility		two six-month extensions
Expansion option on credit facility		$ 150,000,000
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 130,000,000
Nine bank group unsecured revolving credit facility [Member] | $80 million interest rate swap [Member]
Secured and Unsecured Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End				2.02%
Unsecured bank credit facilities - fixed rate, carrying amount				$ 80,000,000
Debt Instrument, Maturity Date, Description				Aug. 15, 2018
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 above 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		3.495%
Line of Credit Facility, Current Borrowing Capacity		$ 45,000,000
Debt instrument, basis spread above LIBOR variable rate		100
Line of credit, facility fee (in basis points)		20
Debt Instrument, Maturity Date, Description		Jul. 30, 2022
Extension option on credit facility		two six-month extensions
Loans Payable, Noncurrent [Abstract]
Unsecured bank credit facilities - variable rate, carrying amount		$ 2,868,000

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