Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Total rental property revenues	$ 114,007	$ 46,454	$ 226,524	$ 91,807
Office
Segment Reporting Information [Line Items]
Total rental property revenues	114,007	46,454	226,524	91,807
Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0	0
Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues	126,442	100,982	253,810	199,722
Operating Segments | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		44,281		87,403
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	47,651	39,805	98,863	79,998
Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	25,429	10,417	49,963	19,356
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	22,599	6,388	45,342	12,734
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	6,331		12,972
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	11,795		23,098
Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	11,879		21,997
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	758	91	1,575	231
Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues	125,084	97,956	248,761	193,719
Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		44,281		87,403
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	46,293	36,779	93,814	73,995
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	25,429	10,417	49,963	19,356
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	22,599	6,388	45,342	12,734
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	6,331		12,972
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	11,795		23,098
Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	11,879		21,997
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues	758	91	1,575	231
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	1,358	3,026	5,049	6,003
Operating Segments | Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total rental property revenues				0
Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	1,358	3,026	5,049	6,003
Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0	0
Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	0	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	0	0	0	0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Less discontinued operations		(44,281)		(87,403)
Less Company's share of rental property revenues from unconsolidated joint ventures	(12,435)	(10,247)	(27,286)	(20,512)
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Less discontinued operations		(44,281)		(87,403)
Less Company's share of rental property revenues from unconsolidated joint ventures	(11,077)	(7,221)	(22,237)	(14,509)
Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Less discontinued operations		0		0
Less Company's share of rental property revenues from unconsolidated joint ventures	$ (1,358)	$ (3,026)	$ (5,049)	$ (6,003)