Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total rental property revenues	$ 249,814	$ 196,244	$ 164,123
Office
Segment Reporting Information [Line Items]
Total rental property revenues	249,814	196,052	162,234
Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	192	1,889
Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues	430,961	410,463	384,637
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	173,583	174,687	134,921
Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	52,769	26,581	14,062
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	39,448	22,964	9,404
Operating Segments | Houston
Segment Reporting Information [Line Items]
Total rental property revenues	136,926	176,823	179,788
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	5,896
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	10,994
Operating Segments | Tempe
Segment Reporting Information [Line Items]
Total rental property revenues	8,902
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	2,443	9,408	46,462
Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues	417,918	400,296	371,714
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	160,540	164,712	125,884
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	52,769	26,581	14,062
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	39,448	22,964	9,404
Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Total rental property revenues	136,926	176,823	179,788
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	5,896
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	10,994
Operating Segments | Office | Tempe
Segment Reporting Information [Line Items]
Total rental property revenues	8,902
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues	2,443	9,216	42,576
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	13,043	9,975	9,037
Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	13,043	9,975	9,037
Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Mixed-Use | Tempe
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	192	3,886
Operating Segments | Other | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Other | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Other | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Other | Houston
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Operating Segments | Other | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Other | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Other | Tempe
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Other | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	192	3,886
Segment Reconciling Items
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	44,220	37,391	37,800
Revenues included in discontinued operations	136,927	176,828	182,714
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	31,177	27,416	26,766
Revenues included in discontinued operations	136,927	176,828	182,714
Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	13,043	9,975	9,037
Revenues included in discontinued operations	0	0	0
Segment Reconciling Items | Other
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	0	0	1,997
Revenues included in discontinued operations	$ 0	$ 0	$ 0