Segment Information (Details) (USD $) In Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 122,086		$ 115,528		$ 354,122		$ 343,796
Total Net Operating Income	76,067		72,189		225,762		220,810
Depreciation and amortization	(36,320)		(34,183)		(103,467)		(100,081)
Impairment of assets held for use	(2,429)		0		(2,429)		0
General and administrative expense	(12,212)		(8,882)		(27,983)		(24,369)
Interest expense	(24,390)		(23,338)		(71,840)		(69,385)
Interest and other income	1,505		1,624		5,253		4,289
Income from continuing operations before disposition of property, condominiums and investment in unconsolidated affiliates and equity in earnings of unconsolidated affiliates	2,221		7,410		25,296		31,264
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,045		11,869		37,290		36,066
Total Net Operating Income	8,053		7,388		23,518		22,848
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,617		3,312		10,560		10,439
Total Net Operating Income	2,097		1,858		6,239		6,307
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,559		3,673		10,801		11,044
Total Net Operating Income	2,023		2,159		6,250		6,699
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,270		9,692		30,450		24,888
Total Net Operating Income	5,698		5,506		16,921		15,013
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,598		14,598		45,576		44,562
Total Net Operating Income	10,247		9,488		30,227		29,422
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,677		2,920		7,614		8,985
Total Net Operating Income	1,378		1,554		3,830		4,891
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,914		5,270		15,549		16,034
Total Net Operating Income	2,896		3,486		9,946		10,577
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	20,051		18,843		59,472		56,174
Total Net Operating Income	13,903		12,527		41,394		38,203
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,378		12,209		36,431		35,485
Total Net Operating Income	7,769		7,631		23,857		23,985
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	17,776		17,830		52,026		53,807
Total Net Operating Income	11,003		10,900		32,194		32,711
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	106,467	[1]	100,216	[1]	307,351	[1]	297,484	[1]
Total Net Operating Income	65,964	[1]	62,497	[1]	195,273	[1]	190,656	[1]
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,976		3,660		11,938		11,477
Total Net Operating Income	2,859		2,492		8,699		8,055
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,151		3,259		8,955		9,324
Total Net Operating Income	2,251		2,459		6,583		6,834
Industrial Total Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	7,127	[1]	6,919	[1]	20,893	[1]	20,801	[1]
Total Net Operating Income	5,110	[1]	4,951	[1]	15,282	[1]	14,889	[1]
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,492		8,105		25,594		24,542
Total Net Operating Income	5,027		4,573		15,149		14,671
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,492	[1]	8,105	[1]	25,594	[1]	24,542	[1]
Total Net Operating Income	5,027	[1]	4,573	[1]	15,149	[1]	14,671	[1]
Residential Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	0		288		284		969
Total Net Operating Income	(34)	[2]	168		58		594
Total Residential Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	0	[1]	288	[1]	284	[1]	969	[1]
Total Net Operating Income	(34)	[1]	168	[1]	58	[1]	594	[1]
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	1,582		0		1,582		0
Total Net Operating Income	$ 897		$ 0		$ 897		$ 0

[1] (1) Net of discontinued operations.

[2] (2) Negative NOI with no corresponding revenues represents expensed real estate taxes and other carrying costs with little or no corresponding revenue.