Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Rental property revenues	$ 109,569	$ 46,575	$ 336,093	$ 138,382
Total rental property revenues	109,569	46,575	336,093	138,382
Office
Segment Reporting Information [Line Items]
Rental property revenues	109,569	46,575	336,093	138,382
Total rental property revenues	109,569	46,575	336,093	138,382
Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Total rental property revenues	0	0	0	0
Operating Segments
Segment Reporting Information [Line Items]
Rental property revenues	121,018	103,147	374,828	302,869
Total rental property revenues	121,018	103,147	374,828	302,869
Operating Segments | Houston
Segment Reporting Information [Line Items]
Rental property revenues		46,046		133,450
Total rental property revenues		46,046		133,450
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	41,507	39,890	140,368	120,115
Total rental property revenues	41,507	39,890	140,368	120,115
Operating Segments | Austin
Segment Reporting Information [Line Items]
Rental property revenues	25,385	10,469	75,348	29,825
Total rental property revenues	25,385	10,469	75,348	29,825
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,296	6,799	68,638	19,533
Total rental property revenues	23,296	6,799	68,638	19,533
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Rental property revenues	6,408		19,380
Total rental property revenues	6,408		19,380
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	11,815		34,913
Total rental property revenues	11,815		34,913
Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	11,692		33,689
Total rental property revenues	11,692		33,689
Operating Segments | Other
Segment Reporting Information [Line Items]
Rental property revenues	915	(57)	2,492	(54)
Total rental property revenues	915	(57)	2,492	(54)
Operating Segments | Office
Segment Reporting Information [Line Items]
Rental property revenues	120,875	99,950	369,636	293,669
Total rental property revenues	120,875	99,950	369,636	293,669
Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Rental property revenues		46,046		133,450
Total rental property revenues		46,046		133,450
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	41,507	36,693	135,319	110,915
Total rental property revenues	41,507	36,693	135,319	110,915
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Rental property revenues	25,385	10,469	75,348	29,825
Total rental property revenues	25,385	10,469	75,348	29,825
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,153	6,799	68,495	19,533
Total rental property revenues	23,153	6,799	68,495	19,533
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Rental property revenues	6,408		19,380
Total rental property revenues	6,408		19,380
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	11,815		34,913
Total rental property revenues	11,815		34,913
Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	11,692		33,689
Total rental property revenues	11,692		33,689
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Rental property revenues	915	(57)	2,492	(54)
Total rental property revenues	915	(57)	2,492	(54)
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	143	3,197	5,192	9,200
Total rental property revenues	143	3,197	5,192	9,200
Operating Segments | Mixed-Use | Houston
Segment Reporting Information [Line Items]
Rental property revenues		0		0
Total rental property revenues		0		0
Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	0	3,197	5,049	9,200
Total rental property revenues	0	3,197	5,049	9,200
Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Total rental property revenues	0	0	0	0
Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	143	0	143	0
Total rental property revenues	143	0	143	0
Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Rental property revenues	0		0
Total rental property revenues	0		0
Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	0		0
Total rental property revenues	0		0
Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	0		0
Total rental property revenues	0		0
Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Total rental property revenues	0	0	0	0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Less discontinued operations		(46,046)		(133,450)
Less Company's share of rental property revenues from unconsolidated joint ventures	(11,449)	(10,526)	(38,735)	(31,037)
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Less discontinued operations		(46,046)		(133,450)
Less Company's share of rental property revenues from unconsolidated joint ventures	(11,306)	(7,329)	(33,543)	(21,837)
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	$ (143)	$ (3,197)	$ (5,192)	$ (9,200)