Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 161,136	$ 152,722	$ 318,446	$ 301,175
Total Net Operating Income	103,858	97,447	203,655	189,510
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(51,240)	(50,443)	(101,548)	(98,608)
Impairments of real estate assets	0	(588)	0	(588)
General and administrative expenses	(8,892)	(8,733)	(20,329)	(19,447)
Interest expense	(22,002)	(21,213)	(43,425)	(42,575)
Other income	979	1,428	2,217	2,827
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	22,703	17,898	40,570	31,119
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	148,984	140,146	294,807	276,489
Total Net Operating Income	96,341	89,402	189,320	174,249
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,611	23,627	50,393	46,604
Total Net Operating Income	15,663	14,321	31,030	28,018
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,398	6,580	10,838	13,030
Total Net Operating Income	3,516	4,249	6,942	8,247
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	0	767	0	1,608
Total Net Operating Income	0	390	0	865
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,142	4,086	8,358	8,183
Total Net Operating Income	2,713	2,698	5,462	5,303
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,050	10,055	23,784	19,890
Total Net Operating Income	7,641	5,789	15,030	11,396
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,770	20,182	43,586	39,787
Total Net Operating Income	15,480	13,999	30,712	27,152
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,948	9,148	21,782	18,068
Total Net Operating Income	6,348	5,346	12,831	10,722
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,719	14,014	29,268	27,771
Total Net Operating Income	8,670	7,988	16,632	15,181
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,541	22,538	48,982	44,029
Total Net Operating Income	18,358	16,326	34,629	31,358
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,203	12,020	20,787	23,763
Total Net Operating Income	6,932	8,265	13,812	15,941
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	18,602	17,129	37,029	33,756
Total Net Operating Income	11,020	10,031	22,240	20,066
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,140	9,366	17,703	18,584
Total Net Operating Income	5,351	5,764	10,063	11,029
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,140	9,366	17,703	18,584
Total Net Operating Income	5,351	5,764	10,063	11,029
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,012	3,210	5,936	6,102
Total Net Operating Income	$ 2,166	$ 2,281	$ 4,272	$ 4,232