Reportable Segments (Segment Net Operating Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 326,063	$ 313,206	$ 260,282
Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	131,564	112,984	105,443
Houston
Segment Reporting Information [Line Items]
Total Net Operating Income			78,590
Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	62,812	62,708	28,418
Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	60,474	58,648	29,865
Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	36,875	34,074	6,067
Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	30,514	29,426	7,130
Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		13,029	3,265
Other
Segment Reporting Information [Line Items]
Total Net Operating Income	3,824	2,337	1,504
Office
Segment Reporting Information [Line Items]
Total Net Operating Income	323,820	309,223	252,871
Office | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	131,564	109,706	98,032
Office | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income			78,590
Office | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	62,812	62,708	28,418
Office | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	60,474	58,648	29,865
Office | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	36,875	34,074	6,067
Office | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	30,514	29,426	7,130
Office | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		13,029	3,265
Office | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	1,581	1,632	1,504
Mixed-Use
Segment Reporting Information [Line Items]
Total Net Operating Income	2,243	3,983	7,411
Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	0	3,278	7,411
Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income			0
Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income		0	0
Mixed-Use | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 2,243	$ 705	$ 0