Reportable Segments (Segment Revenues) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total rental property revenues		$ 657,515	$ 475,212	$ 466,185
Rental property revenues
Segment Reporting Information [Line Items]
Total rental property revenues	$ 628,800	628,751	463,401	455,305
Rental property revenues | Office
Segment Reporting Information [Line Items]
Total rental property revenues		628,751	463,401	455,305
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		681,212	510,898	506,834
Rental property revenues | Operating Segments | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		242,217	206,692	185,734
Rental property revenues | Operating Segments | Austin
Segment Reporting Information [Line Items]
Total rental property revenues		160,196	104,817	101,222
Rental property revenues | Operating Segments | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues		120,214	92,398	92,242
Rental property revenues | Operating Segments | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		54,216	49,822	47,402
Rental property revenues | Operating Segments | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues		51,586	51,238	51,209
Rental property revenues | Operating Segments | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		9,421
Rental property revenues | Operating Segments | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues				24,973
Rental property revenues | Operating Segments | Other
Segment Reporting Information [Line Items]
Total rental property revenues		43,362	5,931	4,052
Rental property revenues | Operating Segments | Office
Segment Reporting Information [Line Items]
Total rental property revenues		676,574	507,174	500,598
Rental property revenues | Operating Segments | Office | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		242,209	206,692	180,497
Rental property revenues | Operating Segments | Office | Austin
Segment Reporting Information [Line Items]
Total rental property revenues		160,196	104,817	101,222
Rental property revenues | Operating Segments | Office | Charlotte
Segment Reporting Information [Line Items]
Total rental property revenues		120,214	92,398	92,242
Rental property revenues | Operating Segments | Office | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		54,216	49,822	47,402
Rental property revenues | Operating Segments | Office | Phoenix
Segment Reporting Information [Line Items]
Total rental property revenues		51,586	51,238	51,209
Rental property revenues | Operating Segments | Office | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		9,421
Rental property revenues | Operating Segments | Office | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues				24,973
Rental property revenues | Operating Segments | Office | Other
Segment Reporting Information [Line Items]
Total rental property revenues		38,732	2,207	3,053
Rental property revenues | Operating Segments | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues		4,638	3,724	6,236
Rental property revenues | Operating Segments | Mixed-Use | Atlanta
Segment Reporting Information [Line Items]
Total rental property revenues		8	0	5,237
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 | Tampa
Segment Reporting Information [Line Items]
Total rental property revenues		0	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 | Dallas
Segment Reporting Information [Line Items]
Total rental property revenues		0
Rental property revenues | Operating Segments | Mixed-Use | Orlando
Segment Reporting Information [Line Items]
Total rental property revenues				0
Rental property revenues | Operating Segments | Mixed-Use | Other
Segment Reporting Information [Line Items]
Total rental property revenues		4,630	3,724	999
Rental property revenues | Segment Reconciling Items
Segment Reporting Information [Line Items]
Total rental property revenues		(52,461)	(47,497)	(51,529)
Rental property revenues | Segment Reconciling Items | Office
Segment Reporting Information [Line Items]
Total rental property revenues		(47,823)	(43,773)	(45,293)
Rental property revenues | Segment Reconciling Items | Mixed-Use
Segment Reporting Information [Line Items]
Total rental property revenues		$ (4,638)	$ (3,724)	$ (6,236)