Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net earnings	$ 47,186,000	$ 38,411,000	$ 37,410,000
Adjustments to reconcile net earnings to net cash flow provided by operating activities:
Depreciation and amortization expense	19,089,000	19,170,000	16,974,000
Impairment charges	9,321,000	12,814,000	17,361,000
(Gains) loss on dispositions of real estate
Continuing operations	(1,041,000)	(6,390,000)	(2,272,000)
Discontinued operations		178,000	(339,000)
Deferred rent receivable	(3,644,000)	(4,516,000)	(4,401,000)
Allowance (recoveries) for uncollectible accounts	205,000	(448,000)	1,089,000
Amortization of above-market and below-market leases	(522,000)	(569,000)	(1,496,000)
Amortization of credit agreement and senior unsecured notes origination costs	771,000	851,000	1,150,000
Accretion expense	3,448,000	4,107,000	4,829,000
Stock-based employee compensation expense	1,350,000	1,426,000	1,090,000
Changes in assets and liabilities:
Accounts receivable	(1,295,000)	(2,383,000)	(1,546,000)
Prepaid expenses and other assets	489,000	445,000	(189,000)
Environmental remediation obligations	(19,798,000)	(24,640,000)	(23,485,000)
Accounts payable and accrued liabilities	1,183,000	(1,582,000)	3,513,000
Net cash flow provided by operating activities	56,742,000	36,874,000	49,688,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Property acquisitions	(214,000,000)	(7,688,000)	(219,192,000)
Capital expenditures	(434,000)	(298,000)	(334,000)
Addition to construction in progress	(1,255,000)	(406,000)	(687,000)
Proceeds from dispositions of real estate
Continuing operations	2,739,000	3,957,000	5,604,000
Discontinued operations		88,000	1,424,000
Deposits for property acquisitions	2,346,000	(2,206,000)	2,844,000
Amortization of investment in direct financing leases	2,511,000	2,001,000	1,666,000
Collection of notes and mortgages receivable	1,876,000	17,532,000	3,647,000
Net cash flow (used in) provided by investing activities	(206,217,000)	12,980,000	(205,028,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings under credit agreements	135,000,000	8,000,000	186,000,000
Repayments under credit agreements	(105,000,000)	(27,000,000)	(67,000,000)
Proceeds from senior unsecured notes	50,000,000		75,000,000
Payments of capital lease obligations	(342,000)	(236,000)	(249,000)
Repayment of mortgage payable		(400,000)	(50,000)
Payments of cash dividends	(39,299,000)	(36,231,000)	(35,150,000)
Payments of loan origination costs	(157,000)		(2,432,000)
Security deposits received (refunded)	247,000	260,000	(187,000)
Payments in settlement of restricted stock units	(1,195,000)	(290,000)	(65,000)
Net cash flow provided by (used in) financing activities	157,094,000	(41,011,000)	155,867,000
Change in cash, cash equivalents and restricted cash	7,619,000	8,843,000	527,000
Cash, cash equivalents and restricted cash at beginning of year	13,194,000	4,351,000	3,824,000
Cash, cash equivalents and restricted cash at end of year	20,813,000	13,194,000	4,351,000
Supplemental disclosures of cash flow information Cash paid during the period for:
Interest	16,435,000	15,707,000	12,643,000
Income taxes	(195,000)	368,000	341,000
Environmental remediation obligations	12,944,000	17,633,000	19,123,000
Non-cash transactions
Dividends declared but not yet paid	12,846,000	9,742,000	15,897,000
Issuance of notes and mortgages receivable related to property dispositions	1,505,000	1,814,000	17,876,000
Accrued construction in progress			$ 268,000
Equity Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net	104,312,000
ATM Program [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net	$ 13,528,000	$ 14,886,000