Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 172,363	$ 180,438
Total Net Operating Income	111,812	121,006
Reconciliation to net income:
Depreciation and amortization	(69,204)	(57,568)
General and administrative expenses	(12,381)	(11,778)
Interest expense	(18,739)	(18,391)
Other income/(loss)	(3,766)	455
Equity in earnings of unconsolidated affiliates	664	522
Net income	8,386	34,246
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	169,141	177,300
Total Net Operating Income	109,456	118,756
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,303	35,483
Total Net Operating Income	23,423	22,670
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,481	6,026
Total Net Operating Income	3,507	4,011
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,757	10,208
Total Net Operating Income	6,022	6,645
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	31,399	30,723
Total Net Operating Income	22,318	21,983
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,932	13,300
Total Net Operating Income	8,556	8,263
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,315	15,282
Total Net Operating Income	9,134	8,971
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	29,097	29,804
Total Net Operating Income	20,859	21,872
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,286	11,049
Total Net Operating Income	8,552	7,581
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	15,571	25,425
Total Net Operating Income	7,085	16,760
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,222	3,138
Total Net Operating Income	$ 2,356	$ 2,250