Reportable Segments (Segment Net Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Total Net Operating Income	$ 75,815	$ 61,621	$ 236,097	$ 180,142
Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		26,408		76,851
Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	25,247	24,346	87,562	71,864
Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	15,074	6,023	44,113	16,978
Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,489	4,905	46,117	14,485
Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income	3,356		10,464
Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,412		21,700
Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,667		24,722
Other
Segment Reporting Information [Line Items]
Total Net Operating Income	570	(61)	1,419	(36)
Office
Segment Reporting Information [Line Items]
Total Net Operating Income	75,770	59,868	232,927	175,041
Office | Houston
Segment Reporting Information [Line Items]
Total Net Operating Income		26,408		76,851
Office | Atlanta
Segment Reporting Information [Line Items]
Total Net Operating Income	25,247	22,593	84,437	66,763
Office | Austin
Segment Reporting Information [Line Items]
Total Net Operating Income	15,074	6,023	44,113	16,978
Office | Charlotte
Segment Reporting Information [Line Items]
Total Net Operating Income	15,489	4,905	46,117	14,485
Office | Orlando
Segment Reporting Information [Line Items]
Total Net Operating Income	3,356		10,464
Office | Tampa
Segment Reporting Information [Line Items]
Total Net Operating Income	7,412		21,700
Office | Phoenix
Segment Reporting Information [Line Items]
Total Net Operating Income	8,667		24,722
Office | Other
Segment Reporting Information [Line Items]
Total Net Operating Income	525	(61)	1,374	(36)
Mixed-Use
Segment Reporting Information [Line Items]
Total Net Operating Income	45	1,753	3,170	5,101
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	0	1,753	3,125	5,101
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	$ 45	$ 0	$ 45	$ 0