Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 188,323	$ 118,706	$ 463,076	$ 352,536
Rental property revenues
Segment Reporting Information [Line Items]
Revenues	180,826	115,719	439,624	343,764
Rental property revenues | Office
Segment Reporting Information [Line Items]
Revenues	180,826	115,719	439,624	343,764
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments
Segment Reporting Information [Line Items]
Revenues	195,616	127,633	482,444	379,249
Rental property revenues | Operating segments | Atlanta
Segment Reporting Information [Line Items]
Revenues	60,486	51,248	177,455	151,479
Rental property revenues | Operating segments | Austin
Segment Reporting Information [Line Items]
Revenues	50,614	26,493	110,521	79,516
Rental property revenues | Operating segments | Charlotte
Segment Reporting Information [Line Items]
Revenues	34,762	23,274	85,258	69,316
Rental property revenues | Operating segments | Dallas
Segment Reporting Information [Line Items]
Revenues	4,274		5,078
Rental property revenues | Operating segments | Phoenix
Segment Reporting Information [Line Items]
Revenues	12,754	12,228	38,562	37,147
Rental property revenues | Operating segments | Tampa
Segment Reporting Information [Line Items]
Revenues	14,335	12,857	40,776	37,556
Rental property revenues | Operating segments | Other
Segment Reporting Information [Line Items]
Revenues	18,391	1,533	24,794	4,235
Rental property revenues | Operating segments | Office
Segment Reporting Information [Line Items]
Revenues	194,515	127,204	479,066	378,252
Rental property revenues | Operating segments | Office | Atlanta
Segment Reporting Information [Line Items]
Revenues	60,486	51,248	177,455	151,479
Rental property revenues | Operating segments | Office | Austin
Segment Reporting Information [Line Items]
Revenues	50,614	26,493	110,521	79,516
Rental property revenues | Operating segments | Office | Charlotte
Segment Reporting Information [Line Items]
Revenues	34,762	23,274	85,258	69,316
Rental property revenues | Operating segments | Office | Dallas
Segment Reporting Information [Line Items]
Revenues	4,274		5,078
Rental property revenues | Operating segments | Office | Phoenix
Segment Reporting Information [Line Items]
Revenues	12,754	12,228	38,562	37,147
Rental property revenues | Operating segments | Office | Tampa
Segment Reporting Information [Line Items]
Revenues	14,335	12,857	40,776	37,556
Rental property revenues | Operating segments | Office | Other
Segment Reporting Information [Line Items]
Revenues	17,290	1,104	21,416	3,238
Rental property revenues | Operating segments | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	1,101	429	3,378	997
Rental property revenues | Operating segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Dallas
Segment Reporting Information [Line Items]
Revenues	0		0
Rental property revenues | Operating segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Revenues	1,101	429	3,378	997
Rental property revenues | Segment reconciling items
Segment Reporting Information [Line Items]
Revenues	(14,790)	(11,914)	(42,820)	(35,485)
Rental property revenues | Segment reconciling items | Office
Segment Reporting Information [Line Items]
Revenues	(13,689)	(11,485)	(39,442)	(34,488)
Rental property revenues | Segment reconciling items | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	$ (1,101)	$ (429)	$ (3,378)	$ (997)