Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 180,185	$ 166,269	$ 526,876	$ 497,988
Total Net Operating Income	118,951	107,649	349,392	324,273
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization	(56,973)	(52,923)	(168,934)	(161,734)
Impairments of real estate assets	(1,445)	0	(1,445)	0
General and administrative expenses	(9,247)	(9,863)	(29,787)	(29,327)
Interest expense	(17,191)	(18,566)	(51,208)	(58,756)
Other income	558	833	2,632	1,884
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates	34,653	27,130	100,650	76,340
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	177,021	162,897	517,079	487,176
Total Net Operating Income	116,637	105,291	342,488	316,782
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,471	33,340	106,027	101,188
Total Net Operating Income	23,600	20,718	68,150	63,681
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,370	5,167	15,956	15,351
Total Net Operating Income	3,367	3,187	10,116	9,544
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,459	12,330	35,077	36,275
Total Net Operating Income	6,930	7,753	21,474	22,566
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	28,945	23,979	81,471	71,760
Total Net Operating Income	21,000	17,042	59,250	51,470
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,309	11,678	38,183	34,360
Total Net Operating Income	7,999	6,633	23,007	19,638
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,719	14,386	44,420	43,721
Total Net Operating Income	8,822	8,482	26,396	25,193
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,524	27,767	90,167	84,013
Total Net Operating Income	21,960	19,525	65,345	59,935
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,152	11,414	33,200	33,420
Total Net Operating Income	6,987	7,602	22,453	22,718
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,072	22,836	72,578	67,088
Total Net Operating Income	15,972	14,349	46,297	42,037
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,164	3,372	9,797	10,812
Total Net Operating Income	$ 2,314	$ 2,358	$ 6,904	$ 7,491