Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 179,417	$ 180,185	$ 538,647	$ 526,876
Total Net Operating Income	118,264	118,951	358,399	349,392
Reconciliation to income before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(57,661)	(56,973)	(171,923)	(168,934)
Impairments of real estate assets	0	(1,445)	0	(1,445)
General and administrative expenses	(9,551)	(9,247)	(30,869)	(29,787)
Interest expense	(17,437)	(17,191)	(53,705)	(51,208)
Other income	818	558	1,735	2,632
Income before disposition of investment properties and activity in unconsolidated affiliates	34,433	34,653	103,637	100,650
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	176,260	177,021	529,073	517,079
Total Net Operating Income	115,928	116,637	351,321	342,488
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,375	36,471	105,635	106,027
Total Net Operating Income	21,443	23,600	65,721	68,150
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,349	5,370	16,753	15,956
Total Net Operating Income	3,372	3,367	10,817	10,116
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,121	11,459	30,228	35,077
Total Net Operating Income	6,471	6,930	19,289	21,474
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	29,927	28,945	91,319	81,471
Total Net Operating Income	21,896	21,000	66,306	59,250
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,352	13,309	40,103	38,183
Total Net Operating Income	8,195	7,999	24,551	23,007
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,982	14,719	45,587	44,420
Total Net Operating Income	8,969	8,822	27,189	26,396
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,432	30,524	89,518	90,167
Total Net Operating Income	22,018	21,960	65,384	65,345
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,262	11,152	33,204	33,200
Total Net Operating Income	7,376	6,987	22,616	22,453
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,460	25,072	76,726	72,578
Total Net Operating Income	16,188	15,972	49,448	46,297
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,157	3,164	9,574	9,797
Total Net Operating Income	$ 2,336	$ 2,314	$ 7,078	$ 6,904