Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 166,860	$ 148,543	$ 331,719	$ 293,779
Total Net Operating Income	109,345	96,173	216,624	188,895
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(55,317)	(47,928)	(108,811)	(94,795)
General and administrative expenses	(8,327)	(8,665)	(19,464)	(19,908)
Interest expense	(19,485)	(21,847)	(40,190)	(43,089)
Other income	534	300	1,051	882
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	26,750	18,033	49,210	31,985
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	163,202	144,842	324,279	286,449
Total Net Operating Income	106,763	93,628	211,491	183,858
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,652	25,611	67,848	50,393
Total Net Operating Income	21,911	15,663	42,963	31,030
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,037	5,398	10,184	10,838
Total Net Operating Income	3,209	3,516	6,357	6,942
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,931	12,050	23,945	23,784
Total Net Operating Income	7,398	7,641	14,813	15,030
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,415	21,770	47,781	43,586
Total Net Operating Income	17,613	15,480	34,428	30,712
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,197	10,948	22,682	21,782
Total Net Operating Income	6,320	6,348	13,005	12,831
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,195	14,719	29,335	29,268
Total Net Operating Income	8,108	8,670	16,711	16,632
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	28,024	25,541	56,246	48,982
Total Net Operating Income	20,156	18,358	40,410	34,629
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,937	10,203	22,006	20,787
Total Net Operating Income	7,687	6,932	15,116	13,812
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,814	18,602	44,252	37,029
Total Net Operating Income	14,361	11,020	27,688	22,240
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,658	3,701	7,440	7,330
Total Net Operating Income	$ 2,582	$ 2,545	$ 5,133	$ 5,037