Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Total rental property revenues	$ 112,517	$ 45,353
Office
Segment Reporting Information [Line Items]
Total rental property revenues	112,517	45,353
Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues	127,367	98,741
Operating Segments | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		43,123
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	51,212	40,194
Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	24,534	8,938
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	22,743	6,345
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	6,641
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	11,303
Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	10,117
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	817	141
Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues	123,676	95,764
Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		43,123
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	47,521	37,217
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	24,534	8,938
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	22,743	6,345
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues	6,641
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	11,303
Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	10,117
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues	817	141
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	3,691	2,977
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	3,691	2,977
Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	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 | Phoenix
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
Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Operating Segments | Other | Houston
Segment Reporting Information [Line Items]
Total rental property revenues		0
Operating Segments | Other | Atlanta
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
Operating Segments | Other | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	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 | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	0
Operating Segments | Other | Other
Segment Reporting Information [Line Items]
Total rental property revenues	0	0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Less Discontinued Operations		(43,123)
Company's share of rental property revenues from unconsolidated joint ventures	(14,850)	(10,265)
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Less Discontinued Operations		(43,123)
Company's share of rental property revenues from unconsolidated joint ventures	(11,159)	(7,288)
Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Less Discontinued Operations		0
Company's share of rental property revenues from unconsolidated joint ventures	(3,691)	(2,977)
Segment Reconciling Items | Other
Segment Reporting Information [Line Items]
Less Discontinued Operations		0
Company's share of rental property revenues from unconsolidated joint ventures	$ 0	$ 0