Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 152,629	$ 144,827	$ 453,804	$ 407,822
Total Net Operating Income	95,246	90,725	284,756	259,809
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(48,287)	(47,338)	(146,895)	(128,050)
Impairments of real estate assets	0	0	(588)	0
General and administrative expenses	(7,526)	(8,969)	(26,973)	(27,948)
Interest expense	(22,348)	(23,672)	(64,923)	(70,826)
Other income	728	1,550	3,555	4,786
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	17,813	12,296	48,932	37,771
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	139,676	132,055	416,165	369,969
Total Net Operating Income	86,776	82,600	261,025	235,640
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,682	22,523	71,286	57,630
Total Net Operating Income	14,808	13,868	42,826	35,936
Office Greenville, SC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	532	827	2,140	2,500
Total Net Operating Income	289	448	1,154	1,400
Office Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	4,218	4,064	12,401	12,067
Total Net Operating Income	2,671	2,597	7,974	7,796
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	10,186	9,644	30,076	28,680
Total Net Operating Income	6,024	5,408	17,420	16,337
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	20,139	15,147	59,926	43,411
Total Net Operating Income	13,590	10,311	40,742	30,028
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,991	8,441	27,059	12,894
Total Net Operating Income	5,087	4,950	15,809	7,099
Office Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	6,439	6,596	19,469	19,487
Total Net Operating Income	4,105	4,257	12,352	12,611
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,259	14,191	42,030	41,835
Total Net Operating Income	8,122	8,001	23,303	23,322
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	21,357	21,737	65,386	64,124
Total Net Operating Income	14,611	14,920	45,969	45,226
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,236	12,006	34,999	35,639
Total Net Operating Income	7,165	7,910	23,106	24,267
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	17,637	16,879	51,393	51,702
Total Net Operating Income	10,304	9,930	30,370	31,618
Total Industrial Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,200	3,208	9,302	9,862
Total Net Operating Income	2,342	2,309	6,614	7,158
Industrial Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	218	210	626	624
Total Net Operating Income	124	122	368	361
Industrial Piedmont Triad, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,982	2,998	8,676	9,238
Total Net Operating Income	2,218	2,187	6,246	6,797
Total Retail Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,753	9,564	28,337	27,991
Total Net Operating Income	6,157	5,837	17,186	17,066
Retail Kansas City, MO [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,753	9,564	28,337	27,991
Total Net Operating Income	6,157	5,837	17,186	17,066
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Total Net Operating Income	$ (29)	$ (21)	$ (69)	$ (55)