Reportable Segments (Segment Net Operating Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 313,206	$ 260,282	$ 241,232
Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		78,590	103,210
Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	112,984	105,443	99,292
Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	62,708	28,418	16,164
Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	58,648	29,865	15,294
Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	34,074	6,067
Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	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	2,337	1,504	7,272
Office
Segment Reporting Information [Line Items]
Total Net Operating Income	309,223	252,871	235,210
Office | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		78,590	103,210
Office | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	109,706	98,032	93,438
Office | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	62,708	28,418	16,164
Office | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	58,648	29,865	15,294
Office | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	34,074	6,067
Office | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	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	1,632	1,504	7,104
Mixed-Use
Segment Reporting Information [Line Items]
Total Net Operating Income	3,983	7,411	5,854
Mixed-Use | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		0	0
Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	3,278	7,411	5,854
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
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	0
Mixed-Use | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	705	0	0
Other
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	168
Other | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		0	0
Other | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Other | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Other | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0
Other | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Other | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Other | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0
Other | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 0	$ 0	$ 168