Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 166,269	$ 150,766	$ 497,988	$ 444,545
Total Net Operating Income	107,649	96,336	324,273	285,231
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(52,923)	(50,963)	(161,734)	(145,758)
General and administrative expenses	(9,863)	(8,990)	(29,327)	(28,898)
Interest expense	(18,566)	(21,357)	(58,756)	(64,446)
Other income	833	379	1,884	1,261
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	27,130	15,405	76,340	47,390
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	162,897	147,087	487,176	433,536
Total Net Operating Income	105,291	93,823	316,782	277,681
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	33,340	25,737	101,188	76,130
Total Net Operating Income	20,718	15,970	63,681	47,000
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,167	5,288	15,351	16,126
Total Net Operating Income	3,187	3,275	9,544	10,217
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,330	11,790	36,275	35,574
Total Net Operating Income	7,753	7,317	22,566	22,347
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,979	22,614	71,760	66,200
Total Net Operating Income	17,042	15,726	51,470	46,438
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,678	11,397	34,360	33,179
Total Net Operating Income	6,633	6,153	19,638	18,984
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,386	14,831	43,721	44,099
Total Net Operating Income	8,482	8,840	25,193	25,472
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	27,767	27,081	84,013	76,063
Total Net Operating Income	19,525	19,018	59,935	53,647
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,414	10,564	33,420	31,351
Total Net Operating Income	7,602	6,909	22,718	20,721
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,836	17,785	67,088	54,814
Total Net Operating Income	14,349	10,615	42,037	32,855
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,372	3,679	10,812	11,009
Total Net Operating Income	$ 2,358	$ 2,513	$ 7,491	$ 7,550