Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Rental property revenues	$ 118,706	$ 113,159	$ 352,536	$ 352,073
Rental Property
Segment Reporting Information [Line Items]
Rental property revenues	115,443	109,569	342,489	336,093
Rental Property | Office
Segment Reporting Information [Line Items]
Rental property revenues	115,443	109,569	342,489	336,093
Rental Property | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments
Segment Reporting Information [Line Items]
Rental property revenues	127,357	121,018	377,974	374,828
Rental Property | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	51,088	41,507	150,917	140,368
Rental Property | Operating Segments | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,415	25,385	79,329	75,348
Rental Property | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,263	23,153	69,342	68,495
Rental Property | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,830	11,692	37,137	33,689
Rental Property | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,228	11,815	37,014	34,913
Rental Property | Operating Segments | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,408		19,380
Rental Property | Operating Segments | Other
Segment Reporting Information [Line Items]
Rental property revenues	1,533	1,058	4,235	2,635
Rental Property | Operating Segments | Office
Segment Reporting Information [Line Items]
Rental property revenues	126,928	120,875	376,977	369,636
Rental Property | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	51,088	41,507	150,917	135,319
Rental Property | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,415	25,385	79,329	75,348
Rental Property | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,263	23,153	69,342	68,495
Rental Property | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,830	11,692	37,137	33,689
Rental Property | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,228	11,815	37,014	34,913
Rental Property | Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,408		19,380
Rental Property | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Rental property revenues	1,104	915	3,238	2,492
Rental Property | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	429	143	997	5,192
Rental Property | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	5,049
Rental Property | Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		0		0
Rental Property | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Rental property revenues	429	143	997	143
Rental Property | Segment Reconciling Items
Segment Reporting Information [Line Items]
Rental property revenues	(11,914)	(11,449)	(35,485)	(38,735)
Rental Property | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Rental property revenues	(11,485)	(11,306)	(34,488)	(33,543)
Rental Property | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	$ (429)	$ (143)	$ (997)	$ (5,192)