Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Rental and other revenues	$ 192,071	$ 187,475	$ 184,070	$ 172,363	$ 181,388	$ 179,417	$ 178,792	$ 180,438	$ 735,979	$ 720,035	$ 702,737
Total net operating income									487,468	477,620	465,849
Reconciliation to net income:
Depreciation and amortization									(254,504)	(229,955)	(227,832)
Impairments of real estate assets									(5,849)	(423)	(1,445)
General and administrative expenses									(44,067)	(40,006)	(39,648)
Interest expense									(81,648)	(71,422)	(69,105)
Other income/(loss)									(2,510)	1,940	2,283
Gains on disposition of property									39,517	37,638	54,157
Equity in earnings of unconsolidated affiliates									3,276	2,238	7,404
Net income	62,346	$ 29,557	$ 41,394	$ 8,386	55,377	$ 35,009	$ 52,998	$ 34,246	141,683	177,630	191,663
Total Assets	5,138,244				4,675,009				5,138,244	4,675,009
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									151,279	141,337	140,323
Total net operating income									97,019	87,503	89,575
Reconciliation to net income:
Total Assets	1,040,869				1,047,850				1,040,869	1,047,850
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									4,650	0	0
Total net operating income									3,791	0	0
Reconciliation to net income:
Total Assets	425,045				0				425,045	0
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									22,154	22,322	21,453
Total net operating income									14,183	14,275	13,612
Reconciliation to net income:
Total Assets	114,030				118,611				114,030	118,611
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									39,494	40,230	45,430
Total net operating income									24,790	25,659	28,128
Reconciliation to net income:
Total Assets	148,832				213,276				148,832	213,276
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									133,867	121,836	111,506
Total net operating income									97,386	88,554	81,204
Reconciliation to net income:
Total Assets	1,045,125				937,732				1,045,125	937,732
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									52,679	53,771	51,236
Total net operating income									32,062	32,841	30,526
Reconciliation to net income:
Total Assets	289,743				306,370				289,743	306,370
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									60,755	61,177	59,103
Total net operating income									36,249	36,233	34,784
Reconciliation to net income:
Total Assets	323,792				329,918				323,792	329,918
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									122,173	118,352	119,254
Total net operating income									88,402	86,053	86,475
Reconciliation to net income:
Total Assets	830,128				792,464				830,128	792,464
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									49,428	45,729	43,959
Total net operating income									33,756	31,276	29,946
Reconciliation to net income:
Total Assets	246,546				248,669				246,546	248,669
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									86,431	102,404	97,524
Total net operating income									50,339	65,819	62,378
Reconciliation to net income:
Total Assets	521,620				522,263				521,620	522,263
Other Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									13,069	12,877	12,949
Total net operating income									9,491	9,407	9,221
Reconciliation to net income:
Total Assets	152,514				157,856				152,514	157,856
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									722,910	707,158	689,788
Total net operating income									477,977	468,213	$ 456,628
Reconciliation to net income:
Total Assets	$ 4,985,730				$ 4,517,153				$ 4,985,730	$ 4,517,153