Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Rental property revenues	$ 113,348	$ 112,517
Total rental property revenues	113,348	112,517
Office
Segment Reporting Information [Line Items]
Rental property revenues	113,348	112,517
Total rental property revenues	113,348	112,517
Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	0	0
Total rental property revenues	0	0
Operating Segments
Segment Reporting Information [Line Items]
Rental property revenues	124,998	127,367
Total rental property revenues	124,998	127,367
Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	49,466	51,212
Total rental property revenues	49,466	51,212
Operating Segments | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,576	24,534
Total rental property revenues	26,576	24,534
Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,041	22,743
Total rental property revenues	23,041	22,743
Operating Segments | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,536	11,303
Total rental property revenues	12,536	11,303
Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,060	10,117
Total rental property revenues	12,060	10,117
Operating Segments | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,641
Total rental property revenues		6,641
Operating Segments | Other
Segment Reporting Information [Line Items]
Rental property revenues	1,319	817
Total rental property revenues	1,319	817
Operating Segments | Office
Segment Reporting Information [Line Items]
Rental property revenues	124,203	123,676
Total rental property revenues	124,203	123,676
Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	49,466	47,521
Total rental property revenues	49,466	47,521
Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,576	24,534
Total rental property revenues	26,576	24,534
Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,041	22,743
Total rental property revenues	23,041	22,743
Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,536	11,303
Total rental property revenues	12,536	11,303
Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,060	10,117
Total rental property revenues	12,060	10,117
Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,641
Total rental property revenues		6,641
Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Rental property revenues	524	817
Total rental property revenues	524	817
Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	795	3,691
Total rental property revenues	795	3,691
Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	0	3,691
Total rental property revenues	0	3,691
Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Rental property revenues	0	0
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	0	0
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	0	0
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	0	0
Total rental property revenues	0	0
Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		0
Total rental property revenues		0
Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Rental property revenues	795	0
Total rental property revenues	795	0
Segment Reconciling Items
Segment Reporting Information [Line Items]
Less: Company's share of rental property revenues from unconsolidated joint ventures	(11,650)	(14,850)
Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Less: Company's share of rental property revenues from unconsolidated joint ventures	(10,855)	(11,159)
Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Less: Company's share of rental property revenues from unconsolidated joint ventures	$ (795)	$ (3,691)