Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 183,805	$ 192,800
Total Net Operating Income	127,616	130,598
Reconciliation to net income:
Depreciation and amortization	(60,927)	(61,150)
General and administrative expenses	(9,952)	(10,930)
Interest expense	(19,768)	(21,277)
Other income	312	69
Gains on disposition of property	18,937	153,067
Equity in earnings of unconsolidated affiliates	637	963
Net income	56,855	191,340
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	180,373	179,416
Total Net Operating Income	125,675	122,673
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,975	37,959
Total Net Operating Income	24,000	25,183
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,134	8,933
Total Net Operating Income	7,252	7,202
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,159	34,497
Total Net Operating Income	24,993	24,753
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,559	13,022
Total Net Operating Income	7,712	8,005
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,618	14,948
Total Net Operating Income	8,889	8,670
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	37,042	32,553
Total Net Operating Income	28,136	23,636
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,462	12,260
Total Net Operating Income	8,053	8,691
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,424	25,244
Total Net Operating Income	16,640	16,533
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,432	13,384
Total Net Operating Income	$ 1,941	$ 7,925