Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 193,898	$ 132,733
Rental property revenues
Segment Reporting Information [Line Items]
Revenues	189,129	123,865
Rental property revenues | Office
Segment Reporting Information [Line Items]
Revenues	189,129	123,865
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating segments
Segment Reporting Information [Line Items]
Revenues	198,694	136,647
Rental property revenues | Operating segments | Atlanta
Segment Reporting Information [Line Items]
Revenues	65,912	57,468
Rental property revenues | Operating segments | Austin
Segment Reporting Information [Line Items]
Revenues	48,747	28,092
Rental property revenues | Operating segments | Charlotte
Segment Reporting Information [Line Items]
Revenues	34,537	23,386
Rental property revenues | Operating segments | Dallas
Segment Reporting Information [Line Items]
Revenues	4,471
Rental property revenues | Operating segments | Phoenix
Segment Reporting Information [Line Items]
Revenues	13,159	13,003
Rental property revenues | Operating segments | Tampa
Segment Reporting Information [Line Items]
Revenues	14,112	12,971
Rental property revenues | Operating segments | Other
Segment Reporting Information [Line Items]
Revenues	17,756	1,727
Rental property revenues | Operating segments | Office
Segment Reporting Information [Line Items]
Revenues	197,397	135,466
Rental property revenues | Operating segments | Office | Atlanta
Segment Reporting Information [Line Items]
Revenues	65,877	57,468
Rental property revenues | Operating segments | Office | Austin
Segment Reporting Information [Line Items]
Revenues	48,747	28,092
Rental property revenues | Operating segments | Office | Charlotte
Segment Reporting Information [Line Items]
Revenues	34,537	23,386
Rental property revenues | Operating segments | Office | Dallas
Segment Reporting Information [Line Items]
Revenues	4,471
Rental property revenues | Operating segments | Office | Phoenix
Segment Reporting Information [Line Items]
Revenues	13,159	13,003
Rental property revenues | Operating segments | Office | Tampa
Segment Reporting Information [Line Items]
Revenues	14,112	12,971
Rental property revenues | Operating segments | Office | Other
Segment Reporting Information [Line Items]
Revenues	16,494	546
Rental property revenues | Operating segments | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	1,297	1,181
Rental property revenues | Operating segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Revenues	35	0
Rental property revenues | Operating segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating segments | Mixed-Use | Dallas
Segment Reporting Information [Line Items]
Revenues	0
Rental property revenues | Operating segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Revenues	1,262	1,181
Rental property revenues | Segment reconciling items
Segment Reporting Information [Line Items]
Revenues	(9,565)	(12,782)
Rental property revenues | Segment reconciling items | Office
Segment Reporting Information [Line Items]
Revenues	(8,268)	(11,601)
Rental property revenues | Segment reconciling items | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	$ (1,297)	$ (1,181)