Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 178,792	$ 177,283	$ 359,230	$ 346,691
Total Net Operating Income	119,129	118,429	240,135	230,441
Reconciliation to income before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(56,694)	(55,816)	(114,262)	(111,961)
General and administrative expenses	(9,540)	(9,050)	(21,318)	(20,540)
Interest expense	(17,877)	(16,154)	(36,268)	(34,017)
Other income	462	1,390	917	2,074
Income before disposition of investment properties and activity in unconsolidated affiliates	35,480	38,799	69,204	65,997
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	175,513	174,031	352,813	340,058
Total Net Operating Income	116,637	116,180	235,393	225,851
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,777	35,347	70,260	69,556
Total Net Operating Income	21,608	22,544	44,278	44,550
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,378	5,284	11,404	10,586
Total Net Operating Income	3,434	3,391	7,445	6,749
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,899	11,823	20,107	23,618
Total Net Operating Income	6,173	7,272	12,818	14,544
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,669	28,836	61,392	52,526
Total Net Operating Income	22,427	21,626	44,410	38,250
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,451	12,436	26,751	24,874
Total Net Operating Income	8,093	7,430	16,356	15,008
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,323	14,852	30,605	29,701
Total Net Operating Income	9,249	8,949	18,220	17,574
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	29,282	30,097	59,086	59,643
Total Net Operating Income	21,494	21,920	43,366	43,385
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,893	11,106	21,942	22,048
Total Net Operating Income	7,659	7,845	15,240	15,466
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,841	24,250	51,266	47,506
Total Net Operating Income	16,500	15,203	33,260	30,325
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,279	3,252	6,417	6,633
Total Net Operating Income	$ 2,492	$ 2,249	$ 4,742	$ 4,590