Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 192,800	$ 172,363
Total Net Operating Income	130,598	111,812
Reconciliation to net income:
Depreciation and amortization	(61,150)	(69,204)
General and administrative expenses	(10,930)	(12,381)
Interest expense	(21,277)	(18,739)
Other income/(loss)	69	(3,766)
Gains on disposition of property	153,067	0
Equity in earnings of unconsolidated affiliates	963	664
Net income	191,340	8,386
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	191,792	169,141
Total Net Operating Income	129,975	109,456
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	37,959	36,303
Total Net Operating Income	25,183	23,423
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,933	0
Total Net Operating Income	7,202	0
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,729	5,481
Total Net Operating Income	2,915	3,507
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	7,647	9,757
Total Net Operating Income	4,387	6,022
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,497	31,399
Total Net Operating Income	24,753	22,318
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,022	13,932
Total Net Operating Income	8,005	8,556
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,948	15,315
Total Net Operating Income	8,670	9,134
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	32,553	29,097
Total Net Operating Income	23,636	20,859
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,260	12,286
Total Net Operating Income	8,691	8,552
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,244	15,571
Total Net Operating Income	16,533	7,085
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	1,008	3,222
Total Net Operating Income	$ 623	$ 2,356