Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Rental property revenues	$ 116,628	$ 119,035	$ 233,830	$ 238,915
Rental Property
Segment Reporting Information [Line Items]
Rental property revenues	113,698	114,007	227,046	226,524
Rental Property | Office
Segment Reporting Information [Line Items]
Rental property revenues	113,698	114,007	227,046	226,524
Rental Property | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments
Segment Reporting Information [Line Items]
Rental property revenues	125,619	126,442	250,617	253,810
Rental Property | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	50,362	47,651	99,828	98,863
Rental Property | Operating Segments | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,339	25,429	52,915	49,963
Rental Property | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,037	22,599	46,078	45,342
Rental Property | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,251	11,795	24,787	23,098
Rental Property | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,247	11,879	24,307	21,997
Rental Property | Operating Segments | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,331		12,972
Rental Property | Operating Segments | Other
Segment Reporting Information [Line Items]
Rental property revenues	1,383	758	2,702	1,575
Rental Property | Operating Segments | Office
Segment Reporting Information [Line Items]
Rental property revenues	124,785	125,084	248,993	248,761
Rental Property | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	50,362	46,293	99,828	93,814
Rental Property | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Rental property revenues	26,339	25,429	52,915	49,963
Rental Property | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	23,037	22,599	46,078	45,342
Rental Property | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	12,251	11,795	24,787	23,098
Rental Property | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	12,247	11,879	24,307	21,997
Rental Property | Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		6,331		12,972
Rental Property | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Rental property revenues	549	758	1,078	1,575
Rental Property | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	834	1,358	1,624	5,049
Rental Property | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Rental property revenues	0	1,358	0	5,049
Rental Property | Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Rental property revenues	0	0	0	0
Rental Property | Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Rental property revenues		0		0
Rental Property | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Rental property revenues	834	0	1,624	0
Rental Property | Segment Reconciling Items
Segment Reporting Information [Line Items]
Rental property revenues	(11,921)	(12,435)	(23,571)	(27,286)
Rental Property | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Rental property revenues	(11,087)	(11,077)	(21,947)	(22,237)
Rental Property | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Rental property revenues	$ (834)	$ (1,358)	$ (1,624)	$ (5,049)