Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total rental property revenues	$ 475,212	$ 466,185	$ 259,211
Rental property revenues
Segment Reporting Information [Line Items]
Total rental property revenues	461,853	446,035	249,814
Rental property revenues | Office
Segment Reporting Information [Line Items]
Total rental property revenues	461,853	446,035	249,814
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Rental property revenues | Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues	509,350	496,270	430,961
Rental property revenues | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	206,129	181,427	173,583
Rental property revenues | Operating Segments | Houston
Segment Reporting Information [Line Items]
Total rental property revenues			136,926
Rental property revenues | Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	104,329	100,939	52,769
Rental property revenues | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	92,454	91,434	39,448
Rental property revenues | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	49,812	47,402	10,994
Rental property revenues | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	50,696	46,186	8,902
Rental property revenues | Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues		24,862	5,896
Rental property revenues | Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues	5,930	4,020	2,443
Rental property revenues | Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues	505,626	490,034	417,918
Rental property revenues | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	206,129	176,190	160,540
Rental property revenues | Operating Segments | Office | Houston
Segment Reporting Information [Line Items]
Total rental property revenues			136,926
Rental property revenues | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	104,329	100,939	52,769
Rental property revenues | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	92,454	91,434	39,448
Rental property revenues | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	49,812	47,402	10,994
Rental property revenues | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	50,696	46,186	8,902
Rental property revenues | Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues		24,862	5,896
Rental property revenues | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues	2,206	3,021	2,443
Rental property revenues | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	3,724	6,236	13,043
Rental property revenues | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues	0	5,237	13,043
Rental property revenues | Operating Segments | Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total rental property revenues			0
Rental property revenues | Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues	0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues		0	0
Rental property revenues | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total rental property revenues	3,724	999	0
Rental property revenues | Segment Reconciling Items
Segment Reporting Information [Line Items]
Total rental property revenues	(47,497)	(50,235)	(44,220)
Revenues included in discontinued operations			(136,927)
Rental property revenues | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Total rental property revenues	(43,773)	(43,999)	(31,177)
Revenues included in discontinued operations			(136,927)
Rental property revenues | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues	$ (3,724)	$ (6,236)	(13,043)
Revenues included in discontinued operations			$ 0