Reportable Segments (Segment Net Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 80,115	$ 59,008	$ 160,281	$ 118,521
Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		25,125		50,443
Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	30,071	23,314	62,316	47,526
Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	14,852	5,763	29,039	10,955
Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,202	4,819	30,627	9,574
Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income	3,318		7,108
Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,451		14,287
Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,838		16,056
Other
Segment Reporting Information [Line Items]
Total Net Operating Income	383	(13)	848	23
Office
Segment Reporting Information [Line Items]
Total Net Operating Income	79,262	57,266	157,155	115,173
Office | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		25,125		50,443
Office | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	29,218	21,572	59,190	44,178
Office | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	14,852	5,763	29,039	10,955
Office | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,202	4,819	30,627	9,574
Office | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income	3,318		7,108
Office | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,451		14,287
Office | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,838		16,056
Office | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	383	(13)	848	23
Mixed-Use
Segment Reporting Information [Line Items]
Total Net Operating Income	853	1,742	3,126	3,348
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	853	1,742	3,126	3,348
Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0	0
Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	0	0	0	0
Mixed-Use | Orlando
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 | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	0		0
Mixed-Use | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 0	$ 0	$ 0	$ 0