Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 157,310	$ 148,453
Total Net Operating Income	99,797	92,063
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(50,308)	(48,165)
General and administrative expenses	(11,437)	(10,714)
Interest expense	(21,423)	(21,362)
Other income	1,238	1,399
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	17,867	13,221
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	145,823	136,343
Total Net Operating Income	92,979	84,847
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,782	22,977
Total Net Operating Income	15,367	13,697
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,440	6,450
Total Net Operating Income	3,426	3,998
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	0	841
Total Net Operating Income	0	475
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,216	4,097
Total Net Operating Income	2,749	2,605
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,734	9,835
Total Net Operating Income	7,389	5,607
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,816	19,605
Total Net Operating Income	15,232	13,153
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,834	8,920
Total Net Operating Income	6,483	5,376
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,549	13,757
Total Net Operating Income	7,962	7,193
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,441	21,491
Total Net Operating Income	16,271	15,032
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,584	11,743
Total Net Operating Income	6,880	7,676
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	18,427	16,627
Total Net Operating Income	11,220	10,035
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,563	9,218
Total Net Operating Income	4,712	5,265
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,563	9,218
Total Net Operating Income	4,712	5,265
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,924	2,892
Total Net Operating Income	$ 2,106	$ 1,951