Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 152,722	$ 132,618	$ 301,175	$ 262,995
Total Net Operating Income	97,447	85,301	189,510	169,084
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(50,443)	(40,624)	(98,608)	(80,712)
Impairments of real estate assets	(588)	0	(588)	0
General and administrative expenses	(8,733)	(8,397)	(19,447)	(18,979)
Interest expense	(21,213)	(23,286)	(42,575)	(47,154)
Other income	1,428	1,617	2,827	3,236
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	17,898	14,611	31,119	25,475
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	140,146	120,328	276,489	237,917
Total Net Operating Income	89,402	77,226	174,249	153,042
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,627	17,933	46,604	35,100
Total Net Operating Income	14,321	11,170	28,018	22,063
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	767	855	1,608	1,706
Total Net Operating Income	390	475	865	971
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,086	4,033	8,183	8,002
Total Net Operating Income	2,698	2,636	5,303	5,198
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,055	9,653	19,890	19,033
Total Net Operating Income	5,789	5,298	11,396	10,927
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	20,182	14,188	39,787	28,258
Total Net Operating Income	13,999	10,030	27,152	19,715
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,148	2,230	18,068	4,451
Total Net Operating Income	5,346	1,070	10,722	2,148
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	6,580	6,516	13,030	12,893
Total Net Operating Income	4,249	4,256	8,247	8,355
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,014	13,951	27,771	27,639
Total Net Operating Income	7,988	7,902	15,181	15,320
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	22,538	21,719	44,029	42,379
Total Net Operating Income	16,326	15,675	31,358	30,299
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,020	11,856	23,763	23,628
Total Net Operating Income	8,265	8,242	15,941	16,355
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	17,129	17,394	33,756	34,828
Total Net Operating Income	10,031	10,472	20,066	21,691
Total Industrial Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,210	3,328	6,102	6,653
Total Net Operating Income	2,283	2,484	4,272	4,848
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	204	211	408	414
Total Net Operating Income	122	120	244	239
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,006	3,117	5,694	6,239
Total Net Operating Income	2,161	2,364	4,028	4,609
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,366	8,962	18,584	18,425
Total Net Operating Income	5,764	5,607	11,029	11,228
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,366	8,962	18,584	18,425
Total Net Operating Income	5,764	5,607	11,029	11,228
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	$ (2)	$ (16)	$ (40)	$ (34)