Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 148,453		$ 130,377
Total Net Operating Income	92,063		83,783
Reconciliation to income from continuing operations before disposition of property and condominiums and acquisition of interest in and equity in earnings of unconsolidated affiliates:
Depreciation and amortization	(48,165)		(40,088)
General and administrative expenses	(10,714)		(10,582)
Interest expense	(21,362)		(23,868)
Other income	1,399		1,619
Income from continuing operations before activity in unconsolidated affiliates	13,221		10,864
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	136,343	[1]	117,589	[1]
Total Net Operating Income	84,847	[1]	75,816	[1]
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,977		17,167
Total Net Operating Income	13,697		10,893
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	841		851
Total Net Operating Income	475		496
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,097		3,969
Total Net Operating Income	2,605		2,562
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,835		9,380
Total Net Operating Income	5,607		5,629
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	19,605		14,070
Total Net Operating Income	13,153		9,685
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,920		2,221
Total Net Operating Income	5,376		1,078
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	6,450		6,377
Total Net Operating Income	3,998		4,099
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,757		13,688
Total Net Operating Income	7,193		7,418
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,491		20,660
Total Net Operating Income	15,032		14,624
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,743		11,772
Total Net Operating Income	7,676		8,113
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	16,627		17,434
Total Net Operating Income	10,035		11,219
Total Industrial Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,892	[1]	3,325	[1]
Total Net Operating Income	1,989	[1]	2,364	[1]
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	204		203
Total Net Operating Income	122		119
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,688		3,122
Total Net Operating Income	1,867		2,245
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,218	[1]	9,463	[1]
Total Net Operating Income	5,265	[1]	5,621	[1]
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,218		9,463
Total Net Operating Income	5,265		5,621
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	$ (38)	[1]	$ (18)	[1]

[1] Net of discontinued operations.