Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 169,408	$ 164,859
Total Net Operating Income	112,012	107,279
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(56,145)	(53,494)
General and administrative expenses	(11,490)	(11,137)
Interest expense	(17,863)	(20,705)
Other income	684	517
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	27,198	22,460
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	166,027	161,077
Total Net Operating Income	109,671	104,728
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,209	33,196
Total Net Operating Income	22,006	21,052
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,302	5,147
Total Net Operating Income	3,358	3,148
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,795	12,014
Total Net Operating Income	7,272	7,415
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,690	23,366
Total Net Operating Income	16,624	16,815
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,438	11,485
Total Net Operating Income	7,578	6,685
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,849	15,140
Total Net Operating Income	8,625	8,603
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	29,546	28,222
Total Net Operating Income	21,465	20,254
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,942	11,069
Total Net Operating Income	7,621	7,429
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,256	21,438
Total Net Operating Income	15,122	13,327
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,381	3,782
Total Net Operating Income	$ 2,341	$ 2,551