Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 187,475	$ 179,417	$ 543,908	$ 538,647
Total Net Operating Income	123,340	118,264	358,664	358,399
Reconciliation to net income:
Depreciation and amortization	(60,850)	(57,661)	(189,514)	(171,923)
Impairments of real estate assets	(5,318)	0	(5,849)	0
General and administrative expenses	(11,717)	(9,551)	(33,658)	(30,869)
Interest expense	(20,527)	(17,437)	(59,622)	(53,705)
Other income/(loss)	174	818	(3,271)	1,735
Gains on disposition of property	3,515	3	10,218	16,975
Equity in earnings of unconsolidated affiliates	940	573	2,369	1,641
Net income	29,557	35,009	79,337	122,253
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	184,194	176,260	534,187	529,073
Total Net Operating Income	120,948	115,928	351,590	351,321
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	39,120	35,375	113,115	105,635
Total Net Operating Income	24,372	21,443	72,614	65,721
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,524	5,349	16,611	16,753
Total Net Operating Income	3,500	3,372	10,715	10,817
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,967	10,121	29,669	30,228
Total Net Operating Income	6,104	6,471	18,336	19,289
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,748	29,927	99,256	91,319
Total Net Operating Income	25,191	21,896	71,717	66,306
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,854	13,352	39,781	40,103
Total Net Operating Income	7,889	8,195	24,452	24,551
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,154	14,982	45,463	45,587
Total Net Operating Income	9,150	8,969	27,179	27,189
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,688	30,432	90,458	89,518
Total Net Operating Income	22,066	22,018	65,363	65,384
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,576	11,262	37,203	33,204
Total Net Operating Income	8,337	7,376	25,499	22,616
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,563	25,460	62,631	76,726
Total Net Operating Income	14,339	16,188	35,715	49,448
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,281	3,157	9,721	9,574
Total Net Operating Income	$ 2,392	$ 2,336	$ 7,074	$ 7,078