Reportable Segments - Segment Revenues (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total rental property revenues		$ 740,340	$ 657,515	$ 475,212
Rental property revenues
Segment Reporting Information [Line Items]
Total rental property revenues	$ 721,900	721,883	628,751	463,401
Rental property revenues | Office
Segment Reporting Information [Line Items]
Total rental property revenues		721,883	628,751	463,401
Rental property revenues | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues		0	0	0
Rental property revenues | Operating Segments
Segment Reporting Information [Line Items]
Total rental property revenues		750,217	681,212	510,898
Rental property revenues | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		256,196	242,217	206,692
Rental property revenues | Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues		210,229	160,196	104,817
Rental property revenues | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues		96,537	120,214	92,398
Rental property revenues | Operating Segments | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		18,143	9,421
Rental property revenues | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues		50,671	54,216	51,238
Rental property revenues | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		54,261	51,586	49,822
Rental property revenues | Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues		64,180	43,362	5,931
Rental property revenues | Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues		744,720	676,574	507,174
Rental property revenues | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		255,594	242,209	206,692
Rental property revenues | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues		210,229	160,196	104,817
Rental property revenues | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues		96,537	120,214	92,398
Rental property revenues | Operating Segments | Office | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		18,143	9,421
Rental property revenues | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues		50,671	54,216	51,238
Rental property revenues | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		54,261	51,586	49,822
Rental property revenues | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues		59,285	38,732	2,207
Rental property revenues | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues		5,497	4,638	3,724
Rental property revenues | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		602	8	0
Rental property revenues | Operating Segments | Mixed-Use | Austin
Segment Reporting Information [Line Items]
Total rental property revenues		0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues		0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		0	0
Rental property revenues | Operating Segments | Mixed-Use | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues		0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		0	0	0
Rental property revenues | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total rental property revenues		4,895	4,630	3,724
Rental property revenues | Segment Reconciling Items
Segment Reporting Information [Line Items]
Total rental property revenues		(28,334)	(52,461)	(47,497)
Rental property revenues | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Total rental property revenues		(22,837)	(47,823)	(43,773)
Rental property revenues | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues		$ (5,497)	$ (4,638)	$ (3,724)