Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 163,736	$ 152,629	$ 482,182	$ 453,804
Total Net Operating Income	103,978	95,246	307,633	284,756
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(54,652)	(48,287)	(156,200)	(146,895)
Impairments of real estate assets	0	0	0	(588)
General and administrative expenses	(9,182)	(7,526)	(29,511)	(26,973)
Interest expense	(21,512)	(22,348)	(64,937)	(64,923)
Other income	1,038	728	3,255	3,555
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	19,670	17,813	60,240	48,932
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	151,294	139,676	446,101	416,165
Total Net Operating Income	96,384	86,776	285,704	261,025
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,737	24,682	76,130	71,286
Total Net Operating Income	15,970	14,808	47,000	42,826
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,288	6,439	16,126	19,469
Total Net Operating Income	3,275	4,105	10,217	12,352
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	0	532	0	2,140
Total Net Operating Income	0	289	0	1,154
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,207	4,218	12,565	12,401
Total Net Operating Income	2,561	2,671	8,023	7,974
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,790	10,186	35,574	30,076
Total Net Operating Income	7,317	6,024	22,347	17,420
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,614	20,139	66,200	59,926
Total Net Operating Income	15,726	13,590	46,438	40,742
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,397	8,991	33,179	27,059
Total Net Operating Income	6,153	5,087	18,984	15,809
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,831	14,259	44,099	42,030
Total Net Operating Income	8,840	8,122	25,472	23,303
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	27,081	21,357	76,063	65,386
Total Net Operating Income	19,018	14,611	53,647	45,969
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,564	11,236	31,351	34,999
Total Net Operating Income	6,909	7,165	20,721	23,106
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	17,785	17,637	54,814	51,393
Total Net Operating Income	10,615	10,304	32,855	30,370
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,461	9,753	27,164	28,337
Total Net Operating Income	5,439	6,157	15,502	17,186
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,461	9,753	27,164	28,337
Total Net Operating Income	5,439	6,157	15,502	17,186
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,981	3,200	8,917	9,302
Total Net Operating Income	$ 2,155	$ 2,313	$ 6,427	$ 6,545