Reportable Segments Table (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Fee Income	$ 1,945,000	$ 1,686,000	$ 5,968,000	$ 5,206,000
Other Revenue from Continuing Operations	153,000	444,000	858,000	593,000
Net operating property income, including discontinued operations	61,621,000	60,816,000	180,142,000	179,643,000
Net operating income in joint venture	6,760,000	6,131,000	20,361,000	18,103,000
General and Administrative Expense	4,368,000	2,976,000	17,301,000	12,405,000
Interest Expense	(7,710,000)	(7,673,000)	(22,457,000)	(23,219,000)
Depreciation, Depletion and Amortization	(31,843,000)	(32,538,000)	(96,192,000)	(103,564,000)
Other Expenses	173,000	170,000	681,000	970,000
Reimbursed Expenses	(795,000)	(686,000)	(2,463,000)	(2,514,000)
Income (Loss) from Equity Method Investments	1,527,000	3,716,000	5,144,000	7,088,000
Loss on sale from discontinued operations	0	0	0	(551,000)
Gain on sale of investment properties	0	37,145,000	13,944,000	37,674,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	11,657,000	53,620,000	42,218,000	68,774,000
Houston [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	26,408,000	26,039,000	76,851,000	76,549,000
Atlanta [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	24,346,000	22,747,000	71,864,000	69,068,000
Austin [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	6,023,000	4,424,000	16,978,000	10,524,000
Charlotte [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	4,905,000	4,072,000	14,485,000	12,026,000
Other [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	(61,000)	3,534,000	(36,000)	11,476,000
Mixed-Use [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	1,753,000	1,492,000	5,101,000	4,343,000
Mixed-Use [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Mixed-Use [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	1,753,000	1,492,000	5,101,000	4,343,000
Mixed-Use [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Mixed-Use [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Mixed-Use [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Office Segment [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	59,873,000	59,329,000	175,042,000	175,332,000
Office Segment [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	26,408,000	26,039,000	76,851,000	76,549,000
Office Segment [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	22,593,000	21,255,000	66,763,000	64,725,000
Office Segment [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	6,023,000	4,424,000	16,978,000	10,524,000
Office Segment [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	4,905,000	4,072,000	14,485,000	12,026,000
Office Segment [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	(56,000)	3,539,000	(35,000)	11,508,000
Other [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	(5,000)	(5,000)	(1,000)	(32,000)
Other [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Other [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Other [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Other [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	0	0	0	0
Other [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net operating property income, including discontinued operations	$ (5,000)	$ (5,000)	$ (1,000)	$ (32,000)