Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total rental property revenues	$ 446,035	$ 249,814	$ 196,244
Office
Segment Reporting Information [Line Items]
Total rental property revenues	446,035	249,814	196,052
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	0	192
Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues	496,270	430,961	410,463
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	181,427	173,583	174,687
Operating Segments | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		136,926	176,823
Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	100,939	52,769	26,581
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	91,434	39,448	22,964
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	24,862	5,896
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	47,402	10,994
Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	46,186	8,902
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	4,020	2,443	9,408
Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues	490,034	417,918	400,296
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	176,190	160,540	164,712
Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		136,926	176,823
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	100,939	52,769	26,581
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	91,434	39,448	22,964
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	24,862	5,896
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	47,402	10,994
Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	46,186	8,902
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues	3,021	2,443	9,216
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	6,236	13,043	9,975
Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	5,237	13,043	9,975
Operating Segments | Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		0	0
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 | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total rental property revenues	999	0	0
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	192
Operating Segments | Other | Atlanta
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
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 | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Other | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Other | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Other | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	192
Segment Reconciling Items
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	50,235	44,220	37,391
Revenues included in discontinued operations		136,927	176,828
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	43,999	31,177	27,416
Revenues included in discontinued operations		136,927	176,828
Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	6,236	13,043	9,975
Revenues included in discontinued operations		0	0
Segment Reconciling Items | Other
Segment Reporting Information [Line Items]
Company's share of rental property revenues from unconsolidated joint ventures	$ 0	0	0
Revenues included in discontinued operations		$ 0	$ 0