Reportable Segments (Segment Net Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 80,578	$ 80,167
Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	32,165	32,244
Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	14,941	14,187
Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,842	15,426
Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,974	7,217
Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,728	6,837
Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		3,790
Other
Segment Reporting Information [Line Items]
Total Net Operating Income	928	466
Office
Segment Reporting Information [Line Items]
Total Net Operating Income	80,090	77,895
Office | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	32,165	29,972
Office | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	14,941	14,187
Office | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,842	15,426
Office | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,974	7,217
Office | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,728	6,837
Office | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		3,790
Office | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	440	466
Mixed-Use
Segment Reporting Information [Line Items]
Total Net Operating Income	488	2,272
Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	0	2,272
Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		0
Mixed-Use | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 488	$ 0