Reportable Segments Reportable Segments (Revenue Reconciliation) (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
Reimbursed Expenses	(795,000)	(686,000)	(2,463,000)	(2,514,000)
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)
Acquisition and merger costs	(1,940,000)	(19,000)	(4,383,000)	(104,000)
Other Expenses	(173,000)	(170,000)	(681,000)	(970,000)
Income (Loss) from Equity Method Investments	1,527,000	3,716,000	$ 5,144,000	7,088,000
Document Fiscal Year Focus			2016
Rental property operating expenses	(37,760,000)	(41,331,000)	$ (112,051,000)	(120,672,000)
Net operating income in joint venture	(6,760,000)	(6,131,000)	(20,361,000)	(18,103,000)
Net operating loss (income) from discontinued operations	0	0	0	14,000
Total consolidated revenues	94,719,000	98,146,000	278,658,000	288,025,000
Revenues including share of unconsolidated joint ventures	105,245,000	107,886,000	309,695,000	315,779,000
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes	0	37,145,000	13,944,000	37,674,000
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	0	6,000	0	(565,000)
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]
Revenues including share of unconsolidated joint ventures	46,046,000	45,117,000	133,450,000	133,326,000
Atlanta [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	39,890,000	43,555,000	120,115,000	126,895,000
Austin [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	10,469,000	7,505,000	29,825,000	18,744,000
Charlotte [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	6,799,000	5,704,000	19,533,000	17,027,000
Rental properties revenues from unconsolidated joint ventures	(10,526,000)	(9,740,000)	(31,037,000)	(27,754,000)
Other [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	2,041,000	6,005,000	6,772,000	19,787,000
Office Segment [Member]
Segment Reporting Information [Line Items]
Total consolidated revenues	92,621,000	96,016,000	271,832,000	282,226,000
Revenues including share of unconsolidated joint ventures	99,950,000	103,099,000	293,669,000	302,779,000
Office Segment [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	46,046,000	45,117,000	133,450,000	133,326,000
Office Segment [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	36,693,000	40,898,000	110,915,000	119,694,000
Office Segment [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	10,469,000	7,505,000	29,825,000	18,744,000
Office Segment [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	6,799,000	5,704,000	19,533,000	17,027,000
Rental properties revenues from unconsolidated joint ventures	(7,329,000)	(7,083,000)	(21,837,000)	(20,553,000)
Office Segment [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	(57,000)	3,875,000	(54,000)	13,988,000
Mixed-Use [Member]
Segment Reporting Information [Line Items]
Total consolidated revenues	0	0	0	0
Revenues including share of unconsolidated joint ventures	3,197,000	2,657,000	9,200,000	7,201,000
Mixed-Use [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Mixed-Use [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	3,197,000	2,657,000	9,200,000	7,201,000
Mixed-Use [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Mixed-Use [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Rental properties revenues from unconsolidated joint ventures	(3,197,000)	(2,657,000)	(9,200,000)	(7,201,000)
Mixed-Use [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Other [Member]
Segment Reporting Information [Line Items]
Total consolidated revenues	2,098,000	2,130,000	6,826,000	5,799,000
Revenues including share of unconsolidated joint ventures	2,098,000	2,130,000	6,826,000	5,799,000
Other [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Other [Member] | Atlanta [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Other [Member] | Austin [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Other [Member] | Charlotte [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	0	0	0	0
Rental properties revenues from unconsolidated joint ventures	0	0	0	0
Other [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenues including share of unconsolidated joint ventures	$ 2,098,000	$ 2,130,000	$ 6,826,000	$ 5,799,000