Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 177,283	$ 166,860	$ 346,691	$ 331,719
Total Net Operating Income	118,429	109,345	230,441	216,624
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(55,816)	(55,317)	(111,961)	(108,811)
General and administrative expenses	(9,050)	(8,327)	(20,540)	(19,464)
Interest expense	(16,154)	(19,485)	(34,017)	(40,190)
Other income	1,390	534	2,074	1,051
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	38,799	26,750	65,997	49,210
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	174,031	163,202	340,058	324,279
Total Net Operating Income	116,180	106,763	225,851	211,491
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,347	34,652	69,556	67,848
Total Net Operating Income	22,544	21,911	44,550	42,963
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,284	5,037	10,586	10,184
Total Net Operating Income	3,391	3,209	6,749	6,357
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,823	11,931	23,618	23,945
Total Net Operating Income	7,272	7,398	14,544	14,813
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	28,836	24,415	52,526	47,781
Total Net Operating Income	21,626	17,613	38,250	34,428
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,436	11,197	24,874	22,682
Total Net Operating Income	7,430	6,320	15,008	13,005
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,852	14,195	29,701	29,335
Total Net Operating Income	8,949	8,108	17,574	16,711
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,097	28,024	59,643	56,246
Total Net Operating Income	21,920	20,156	43,385	40,410
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,106	10,937	22,048	22,006
Total Net Operating Income	7,845	7,687	15,466	15,116
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,250	22,814	47,506	44,252
Total Net Operating Income	15,203	14,361	30,325	27,688
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,252	3,658	6,633	7,440
Total Net Operating Income	$ 2,249	$ 2,582	$ 4,590	$ 5,133