Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 142,020	$ 116,628	$ 274,753	$ 233,830
Rental property revenues
Segment Reporting Information [Line Items]
Revenues	134,933	114,337	258,798	228,045
Rental property revenues | Office
Segment Reporting Information [Line Items]
Revenues	134,933	114,337	258,798	228,045
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	150,178		286,828
Rental property revenues | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Revenues	59,499	50,521	116,969	100,229
Rental property revenues | Operating Segments | Austin
Segment Reporting Information [Line Items]
Revenues	31,815	26,407	59,907	53,167
Rental property revenues | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Revenues	27,110	23,053	50,496	46,040
Rental property revenues | Operating Segments | Dallas
Segment Reporting Information [Line Items]
Revenues	804		804
Rental property revenues | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Revenues	12,805	12,710	25,808	24,822
Rental property revenues | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Revenues	13,471	12,251	26,441	24,796
Rental property revenues | Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	4,674	1,383	6,403	2,702
Rental property revenues | Operating Segments | Office
Segment Reporting Information [Line Items]
Revenues	149,085		284,551
Rental property revenues | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Revenues	59,499	50,521	116,969	100,229
Rental property revenues | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Revenues	31,815	26,407	59,907	53,167
Rental property revenues | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Revenues	27,110	23,053	50,496	46,040
Rental property revenues | Operating Segments | Office | Dallas
Segment Reporting Information [Line Items]
Revenues	804		804
Rental property revenues | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Revenues	12,805	12,710	25,808	24,822
Rental property revenues | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Revenues	13,471	12,251	26,441	24,796
Rental property revenues | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Revenues	3,581	549	4,126	1,078
Rental property revenues | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	1,093		2,277
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	1,093	834	2,277	1,624
Rental property revenues | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	(15,245)	(11,988)	(28,030)	(23,711)
Total segment revenues		126,325		251,756
Rental property revenues | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Revenues	(14,152)	(11,154)	(25,753)	(22,087)
Total segment revenues		125,491		250,132
Rental property revenues | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	$ (1,093)	(834)	$ (2,277)	(1,624)
Total segment revenues		$ 834		$ 1,624