Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 138,515		$ 126,728		$ 274,631		$ 250,762
Total Net Operating Income	88,882		80,844		176,259		160,696
Reconciliation to income from continuing operations before disposition of condominiums and equity in earnings/(losses) of unconsolidated affiliates:
Depreciation and amortization	(42,394)		(38,991)		(84,208)		(75,687)
Impairments of real estate assets	(1,066)		0		(1,066)		0
General and administrative expenses	(8,397)		(8,900)		(18,979)		(18,573)
Interest expense	(23,286)		(24,372)		(47,154)		(49,049)
Other income	1,617		764		3,236		2,994
Income from continuing operations before disposition of property and condominiums and equity in earnings of unconsolidated affiliates	15,356		9,345		28,088		20,381
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	124,205	[1]	112,403	[1]	245,673	[1]	222,265	[1]
Total Net Operating Income	79,365	[1]	71,763	[1]	157,411	[1]	142,405	[1]
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	18,303		15,331		35,837		30,238
Total Net Operating Income	11,446		9,785		22,616		19,511
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,130		3,488		6,359		6,991
Total Net Operating Income	1,680		2,050		3,569		4,181
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,035		3,559		8,005		7,161
Total Net Operating Income	2,637		2,184		5,200		4,516
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,657		9,248		19,040		18,503
Total Net Operating Income	5,300		5,297		10,931		10,851
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,194		14,317		28,269		28,178
Total Net Operating Income	10,034		9,831		19,722		19,482
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,231		2,219		4,453		4,377
Total Net Operating Income	1,070		1,056		2,149		2,120
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	7,086		5,071		13,977		10,149
Total Net Operating Income	4,568		3,218		8,923		6,449
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,957		9,110		27,649		18,194
Total Net Operating Income	7,904		4,624		15,327		8,904
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,726		20,395		42,394		40,169
Total Net Operating Income	15,682		14,381		30,312		28,339
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,861		12,092		23,637		23,599
Total Net Operating Income	8,245		8,532		16,361		16,412
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	18,025		17,573		36,053		34,706
Total Net Operating Income	10,799		10,805		22,301		21,640
Total Industrial Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,344	[1]	5,253	[1]	10,527	[1]	10,504	[1]
Total Net Operating Income	3,924	[1]	3,856	[1]	7,650	[1]	7,642	[1]
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,226		2,167		4,286		4,254
Total Net Operating Income	1,559		1,549		3,039		3,048
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,118		3,086		6,241		6,250
Total Net Operating Income	2,365		2,307		4,611		4,594
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,966	[1]	9,072	[1]	18,431	[1]	17,993	[1]
Total Net Operating Income	5,609	[1]	5,320	[1]	11,232	[1]	10,853	[1]
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,966		9,072		18,431		17,993
Total Net Operating Income	5,609		5,320		11,232		10,853
Total Residential Segment [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	0	[1]	(65)	[1]	0	[1]	(145)	[1]
Residential Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	0		(65)		0		(145)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	$ (16)	[1]	$ (30)	[1]	$ (34)	[1]	$ (59)	[1]

[1] Net of discontinued operations.