Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 184,070	$ 178,792	$ 356,433	$ 359,230
Total Net Operating Income	123,512	119,129	235,324	240,135
Reconciliation to net income:
Depreciation and amortization	(59,460)	(56,694)	(128,664)	(114,262)
Impairments of real estate assets	(531)	0	(531)	0
General and administrative expenses	(9,560)	(9,540)	(21,941)	(21,318)
Interest expense	(20,356)	(17,877)	(39,095)	(36,268)
Other income/(loss)	321	462	(3,445)	917
Gains on disposition of property	6,703	16,972	6,703	16,972
Equity in earnings of unconsolidated affiliates	765	546	1,429	1,068
Net income	41,394	52,998	49,780	87,244
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	180,852	175,513	349,993	352,813
Total Net Operating Income	121,186	116,637	230,642	235,393
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	37,692	34,777	73,995	70,260
Total Net Operating Income	24,819	21,608	48,242	44,278
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	5,606	5,378	11,087	11,404
Total Net Operating Income	3,708	3,434	7,215	7,445
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,945	9,899	19,702	20,107
Total Net Operating Income	6,210	6,173	12,232	12,818
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	33,109	30,669	64,508	61,392
Total Net Operating Income	24,208	22,427	46,526	44,410
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,995	13,451	26,927	26,751
Total Net Operating Income	8,007	8,093	16,563	16,356
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,994	15,323	30,309	30,605
Total Net Operating Income	8,895	9,249	18,029	18,220
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	30,673	29,282	59,770	59,086
Total Net Operating Income	22,438	21,494	43,297	43,366
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,341	10,893	24,627	21,942
Total Net Operating Income	8,610	7,659	17,162	15,240
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,497	25,841	39,068	51,266
Total Net Operating Income	14,291	16,500	21,376	33,260
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,218	3,279	6,440	6,417
Total Net Operating Income	$ 2,326	$ 2,492	$ 4,682	$ 4,742