Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 147,294		$ 123,418		$ 415,961		$ 368,029
Total Net Operating Income	91,929		77,514		264,159		233,927
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	(48,124)		(37,041)		(130,390)		(110,656)
General and administrative expenses	(8,969)		(9,725)		(27,948)		(28,298)
Interest expense	(23,672)		(23,612)		(70,826)		(72,661)
Other income	1,550		1,916		4,786		4,910
Income from continuing operations before disposition of property and condominiums and acquisition of controlling interest in and equity in earnings/(losses) of unconsolidated affiliates	12,714		9,052		39,781		27,222
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	134,522	[1]	110,902	[1]	378,108	[1]	330,854	[1]
Total Net Operating Income	83,804	[1]	69,521	[1]	239,990	[1]	210,450	[1]
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,523		13,704		57,630		43,220
Total Net Operating Income	13,868		8,408		35,936		27,346
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,699		3,296		9,056		10,286
Total Net Operating Income	1,301		1,806		4,869		5,986
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,064		4,014		12,067		11,173
Total Net Operating Income	2,597		2,546		7,796		7,060
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,644		9,103		28,680		27,603
Total Net Operating Income	5,408		5,378		16,337		16,226
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,147		14,285		43,411		42,456
Total Net Operating Income	10,311		9,772		30,028		29,247
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,441		2,217		12,894		6,594
Total Net Operating Income	4,950		947		7,099		3,066
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	6,792		5,176		19,932		14,532
Total Net Operating Income	4,373		3,095		12,823		9,103
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,191		9,482		41,835		27,671
Total Net Operating Income	8,001		4,949		23,322		13,851
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,737		20,578		64,124		60,737
Total Net Operating Income	14,920		13,939		45,226		42,267
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,006		11,848		35,639		35,442
Total Net Operating Income	7,910		7,863		24,267		24,270
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	17,278		17,199		52,840		51,140
Total Net Operating Income	10,165		10,818		32,287		32,028
Total Industrial Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,208	[1]	3,330	[1]	9,862	[1]	9,999	[1]
Total Net Operating Income	2,309	[1]	2,356	[1]	7,158	[1]	7,195	[1]
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	210		189		624		610
Total Net Operating Income	122		100		361		347
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,998		3,141		9,238		9,389
Total Net Operating Income	2,187		2,256		6,797		6,848
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,564	[1]	9,186	[1]	27,991	[1]	27,176	[1]
Total Net Operating Income	5,837	[1]	5,676	[1]	17,066	[1]	16,525	[1]
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,564		9,186		27,991		27,176
Total Net Operating Income	5,837		5,676		17,066		16,525
Total Residential Segment [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	0	[1]	(33)	[1]	0	[1]	(178)	[1]
Residential Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	0		(33)		0		(178)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	$ (21)	[1]	$ (6)	[1]	$ (55)	[1]	$ (65)	[1]

[1] Net of discontinued operations.