Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 132,733	$ 117,202
Rental property revenues
Segment Reporting Information [Line Items]
Revenues	123,345	113,348
Rental property revenues | Office
Segment Reporting Information [Line Items]
Revenues	123,345	113,348
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating Segments
Segment Reporting Information [Line Items]
Revenues	136,127
Rental property revenues | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Revenues	57,468	49,466
Rental property revenues | Operating Segments | Austin
Segment Reporting Information [Line Items]
Revenues	27,556	26,576
Rental property revenues | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Revenues	23,402	23,041
Rental property revenues | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Revenues	12,971	12,536
Rental property revenues | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Revenues	13,003	12,060
Rental property revenues | Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	1,727	1,319
Rental property revenues | Operating Segments | Office
Segment Reporting Information [Line Items]
Revenues	134,946
Rental property revenues | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Revenues	57,468	49,466
Rental property revenues | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Revenues	27,556	26,576
Rental property revenues | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Revenues	23,402	23,041
Rental property revenues | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Revenues	12,971	12,536
Rental property revenues | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Revenues	13,003	12,060
Rental property revenues | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Revenues	546	524
Rental property revenues | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	1,181
Rental property revenues | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Revenues	0	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 | Tampa
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Revenues	0	0
Rental property revenues | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Revenues	1,181	795
Rental property revenues | Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	(12,782)	(11,650)
Total segment revenues		124,998
Rental property revenues | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Revenues	(11,601)	(10,855)
Total segment revenues		124,203
Rental property revenues | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Revenues	$ (1,181)	(795)
Total segment revenues		$ 795