Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net earnings	$ 38,411,000	$ 37,410,000	$ 23,418,000
Adjustments to reconcile net earnings to net cash flow provided by operating activities:
Depreciation and amortization expense	19,170,000	16,974,000	10,549,000
Impairment charges	12,814,000	17,361,000	21,534,000
(Gains) loss on dispositions of real estate
Continuing operations	(6,418,000)	(2,272,000)	(1,223,000)
Discontinued operations	205,000	(339,000)	(8,995,000)
Deferred rent receivable, net of allowance	(4,516,000)	(4,401,000)	(4,156,000)
(Recoveries) allowance for uncollectible accounts	(448,000)	1,089,000	1,278,000
Amortization of above-market and below-market leases	(569,000)	(1,496,000)	(28,000)
Amortization of credit agreement and senior unsecured notes origination costs	851,000	1,150,000	1,068,000
Accretion expense	4,107,000	4,829,000	3,046,000
Stock-based employee compensation expense	1,426,000	1,090,000	917,000
Changes in assets and liabilities:
Accounts receivable	(2,382,000)	(1,546,000)	(730,000)
Prepaid expenses and other assets	445,000	(189,000)	3,934,000
Environmental remediation obligations	(24,640,000)	(23,485,000)	(16,368,000)
Accounts payable and accrued liabilities	(1,582,000)	3,513,000	(5,007,000)
Net cash flow provided by operating activities	36,874,000	49,688,000	29,237,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Property acquisitions	(7,688,000)	(219,192,000)	(17,098,000)
Capital expenditures	(298,000)	(334,000)	(140,000)
Addition to construction in progress	(406,000)	(687,000)
Proceeds from dispositions of real estate
Continuing operations	3,957,000	5,604,000	4,776,000
Discontinued operations	88,000	1,424,000	15,289,000
Deposits for property acquisitions	(2,206,000)	2,844,000	16,226,000
Change in restricted cash	(262,000)	304,000	287,000
Amortization of investment in direct financing leases	2,001,000	1,666,000	1,382,000
Collection of notes and mortgages receivable	17,532,000	3,647,000	2,783,000
Net cash flow provided by (used in) investing activities	12,718,000	(204,724,000)	23,505,000
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings under credit agreements	8,000,000	186,000,000	3,000,000
Repayments under credit agreements	(27,000,000)	(67,000,000)	(36,000,000)
Borrowings under senior unsecured notes		75,000,000
Payments of capital lease obligations	(236,000)	(249,000)	(255,000)
Repayment of mortgage payable	(400,000)	(50,000)	(50,000)
Payments of cash dividends	(36,231,000)	(35,150,000)	(28,675,000)
Payments of loan origination costs		(2,432,000)
Security deposits received (refunded)	260,000	(187,000)	314,000
Cash paid in settlement of restricted stock units	(290,000)	(65,000)
Proceeds from issuance of common stock, net	14,886,000
Net cash flow (used in) provided by financing activities	(41,011,000)	155,867,000	(61,666,000)
Change in cash and cash equivalents	8,581,000	831,000	(8,924,000)
Cash and cash equivalents at beginning of year	3,942,000	3,111,000	12,035,000
Cash and cash equivalents at end of year	12,523,000	3,942,000	3,111,000
Supplemental disclosures of cash flow information Cash paid during the period for:
Interest	15,707,000	12,643,000	8,735,000
Income taxes	368,000	341,000	316,000
Environmental remediation obligations	17,633,000	19,123,000	13,448,000
Non-cash transactions
Issuance of notes and mortgages receivable related to property dispositions	$ 1,814,000	17,876,000	8,278,000
Mortgage payable, net related to property acquisition			$ 390,000
Accrued construction in progress		$ 268,000