Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 180,438	$ 169,408
Total Net Operating Income	121,006	112,012
Reconciliation to income before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(57,568)	(56,145)
General and administrative expenses	(11,778)	(11,490)
Interest expense	(18,391)	(17,863)
Other income	455	684
Income before disposition of investment properties and activity in unconsolidated affiliates	33,724	27,198
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	177,300	166,027
Total Net Operating Income	118,756	109,671
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,483	34,209
Total Net Operating Income	22,670	22,006
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	6,026	5,302
Total Net Operating Income	4,011	3,358
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,208	11,795
Total Net Operating Income	6,645	7,272
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,723	23,690
Total Net Operating Income	21,983	16,624
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,300	12,438
Total Net Operating Income	8,263	7,578
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,282	14,849
Total Net Operating Income	8,971	8,625
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	29,804	29,546
Total Net Operating Income	21,872	21,465
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,049	10,942
Total Net Operating Income	7,581	7,621
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,425	23,256
Total Net Operating Income	16,760	15,122
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,138	3,381
Total Net Operating Income	$ 2,250	$ 2,341