CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 45,619,000	$ 17,215,000	$ 23,691,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	25,091,000	23,008,000	22,198,000
Amortization of debt premium	(139,000)	(225,000)	(225,000)
Stock-based compensation expense	538,000	481,000	427,000
Hurricane related expenses	4,967,000
Hurricane insurance recoveries	(4,967,000)
Hurricane insurance recovery proceeds in excess of damaged property write-downs	(2,033,000)
Gain on Arlington transaction	(27,196,000)
Gain on property exchange			(8,742,000)
Changes in assets and liabilities:
Rent receivable	(1,490,000)	(1,204,000)	(290,000)
Accrued expenses and other liabilities	441,000	3,000	(184,000)
Tenant reserves, deposits and deferred and prepaid rents	4,989,000	1,377,000	1,781,000
Accrued interest	(86,000)	122,000	(41,000)
Leasing costs paid	(769,000)	(627,000)	(556,000)
Other, net	1,040,000	583,000	119,000
Net cash provided by operating activities	46,005,000	40,733,000	38,178,000
Cash flows from investing activities:
Investments in LLCs	(532,000)	(5,454,000)	(667,000)
Repayments of advances made to LLCs	216,000	851,000	306,000
Advances made to LLCs			(22,795,000)
Cash distributions in excess of income from LLCs	1,187,000	1,362,000	224,000
Cash distribution of refinancing proceeds from LLCs			1,045,000
Additions to real estate investments, net	(15,313,000)	(11,204,000)	(5,257,000)
Cash proceeds received from divestiture of property, net	65,220,000	0	0
Cash received for property exchange			2,000,000
Hurricane insurance recoveries for damaged real estate property	4,967,000
Hurricane insurance recovery proceeds in excess of damaged property write-downs	2,033,000
Hurricane remediation expenses paid	(1,387,000)
Deposits on real estate assets			(150,000)
Net cash paid for acquisition of properties	(9,040,000)	(60,389,000)	(16,765,000)
Cash paid to acquire minority interests in majority-owned LLCs	(7,890,000)		(2,250,000)
Net cash provided by/(used in) investing activities	39,461,000	(74,834,000)	(44,309,000)
Cash flows from financing activities:
Net borrowings on line of credit		59,350,000	52,400,000
Net repayments on line of credit	(20,450,000)
Proceeds from mortgage notes payable	22,600,000		5,200,000
Repayments of mortgage notes payable	(61,021,000)	(3,230,000)	(17,826,000)
Financing costs paid	(446,000)	(307,000)	(1,114,000)
Dividends paid	(36,054,000)	(35,138,000)	(34,089,000)
Partial settlement of dividends equivalent rights		(30,000)	(75,000)
Issuance of shares of beneficial interest, net	9,362,000	13,492,000	1,668,000
Net cash (used in)/provided by financing activities	(86,009,000)	34,137,000	6,164,000
(Decrease)/increase in cash and cash equivalents	(543,000)	36,000	33,000
Cash and cash equivalents, beginning of period	3,930,000	3,894,000	3,861,000
Cash and cash equivalents, end of period	3,387,000	3,930,000	3,894,000
Supplemental disclosures of cash flow information:
Interest paid	$ 9,692,000	8,895,000	8,025,000
Supplemental disclosures of non-cash transactions:
Financing assumed in acquisition		$ 7,499,000
Net assets acquired in property exchange			9,886,000
Net assets relinquished in property exchange			$ (3,144,000)