Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 164,859	$ 145,236
Total Net Operating Income	107,279	92,722
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(53,494)	(46,867)
General and administrative expenses	(11,137)	(11,243)
Interest expense	(20,705)	(21,242)
Other income	517	582
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	22,460	13,952
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	161,077	141,607
Total Net Operating Income	104,728	90,230
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	33,196	24,782
Total Net Operating Income	21,052	15,367
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,147	5,440
Total Net Operating Income	3,148	3,426
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,014	11,734
Total Net Operating Income	7,415	7,389
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,366	21,816
Total Net Operating Income	16,815	15,232
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,485	10,834
Total Net Operating Income	6,685	6,483
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,140	14,549
Total Net Operating Income	8,603	7,962
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	28,222	23,441
Total Net Operating Income	20,254	16,271
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,069	10,584
Total Net Operating Income	7,429	6,880
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,438	18,427
Total Net Operating Income	13,327	11,220
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,782	3,629
Total Net Operating Income	$ 2,551	$ 2,492