Reportable Segments - (Segment Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total Revenues	$ 183,380	$ 188,323	$ 557,193	$ 463,076
Rental property revenues
Segment Reporting Information [Line Items]
Total Revenues	179,024	180,826	543,252	439,624
Rental property revenues | Office
Segment Reporting Information [Line Items]
Total Revenues	179,024	180,826	543,252	439,624
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Total Revenues	0	0	0	0
Rental property revenues | Operating segments
Segment Reporting Information [Line Items]
Total Revenues	185,246	195,616	565,188	482,444
Rental property revenues | Operating segments | Atlanta
Segment Reporting Information [Line Items]
Total Revenues	64,852	60,486	193,351	177,455
Rental property revenues | Operating segments | Austin
Segment Reporting Information [Line Items]
Total Revenues	52,008	50,614	155,009	110,521
Rental property revenues | Operating segments | Charlotte
Segment Reporting Information [Line Items]
Total Revenues	20,370	34,762	75,411	85,258
Rental property revenues | Operating segments | Dallas
Segment Reporting Information [Line Items]
Total Revenues	4,601	4,274	13,559	5,078
Rental property revenues | Operating segments | Phoenix
Segment Reporting Information [Line Items]
Total Revenues	12,689	12,754	37,932	38,562
Rental property revenues | Operating segments | Tampa
Segment Reporting Information [Line Items]
Total Revenues	13,179	14,335	40,203	40,776
Rental property revenues | Operating segments | Other
Segment Reporting Information [Line Items]
Total Revenues	17,547	18,391	49,723	24,794
Rental property revenues | Operating segments | Office
Segment Reporting Information [Line Items]
Total Revenues	183,878	194,515	561,218	479,066
Rental property revenues | Operating segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total Revenues	64,677	60,486	193,012	177,455
Rental property revenues | Operating segments | Office | Austin
Segment Reporting Information [Line Items]
Total Revenues	52,008	50,614	155,009	110,521
Rental property revenues | Operating segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total Revenues	20,370	34,762	75,411	85,258
Rental property revenues | Operating segments | Office | Dallas
Segment Reporting Information [Line Items]
Total Revenues	4,601	4,274	13,559	5,078
Rental property revenues | Operating segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total Revenues	12,689	12,754	37,932	38,562
Rental property revenues | Operating segments | Office | Tampa
Segment Reporting Information [Line Items]
Total Revenues	13,179	14,335	40,203	40,776
Rental property revenues | Operating segments | Office | Other
Segment Reporting Information [Line Items]
Total Revenues	16,354	17,290	46,092	21,416
Rental property revenues | Operating segments | Mixed-Use
Segment Reporting Information [Line Items]
Total Revenues	1,368	1,101	3,970	3,378
Rental property revenues | Operating segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total Revenues	175	0	339	0
Rental property revenues | Operating segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Dallas
Segment Reporting Information [Line Items]
Total Revenues	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total Revenues	0	0	0	0
Rental property revenues | Operating segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total Revenues	1,193	1,101	3,631	3,378
Rental property revenues | Segment reconciling items
Segment Reporting Information [Line Items]
Total Revenues	(6,222)	(14,790)	(21,936)	(42,820)
Rental property revenues | Segment reconciling items | Office
Segment Reporting Information [Line Items]
Total Revenues	(4,854)	(13,689)	(17,966)	(39,442)
Rental property revenues | Segment reconciling items | Mixed-Use
Segment Reporting Information [Line Items]
Total Revenues	$ (1,368)	$ (1,101)	$ (3,970)	$ (3,378)