Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Abstract]
Rental and other revenues	$ 179,904	$ 181,043	$ 183,153	$ 192,800	$ 192,071	$ 187,475	$ 184,070	$ 172,363	$ 736,900	$ 735,979	$ 720,035
Total net operating income									505,075	487,468	477,620
Reconciliation to net income:
Depreciation and amortization									(241,585)	(254,504)	(229,955)
Impairments of real estate assets									(1,778)	(5,849)	(423)
General and administrative expenses									(41,031)	(44,067)	(40,006)
Interest expense									(80,962)	(81,648)	(71,422)
Other income/(loss)									(1,707)	(2,510)	1,940
Gains on disposition of property									215,897	39,517	37,638
Equity in earnings of unconsolidated affiliates									4,005	3,276	2,238
Net income	85,287	$ 42,331	$ 38,956	$ 191,340	62,346	$ 29,557	$ 41,394	$ 8,386	357,914	141,683	177,630
Total Assets	5,209,417				5,138,244				5,209,417	5,138,244
Office Atlanta, GA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									146,704	151,279	141,337
Total net operating income									95,448	97,019	87,503
Reconciliation to net income:
Total Assets	1,011,807				1,040,869				1,011,807	1,040,869
Office Charlotte, NC [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									35,733	4,650	0
Total net operating income									28,431	3,791	0
Reconciliation to net income:
Total Assets	443,051				425,045				443,051	425,045
Office Nashville, TN [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									138,089	133,867	121,836
Total net operating income									99,901	97,386	88,554
Reconciliation to net income:
Total Assets	1,191,219				1,045,125				1,191,219	1,045,125
Office Orlando, FL [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									49,459	52,679	53,771
Total net operating income									29,546	32,062	32,841
Reconciliation to net income:
Total Assets	289,129				289,743				289,129	289,743
Office Pittsburgh, PA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									58,518	60,755	61,177
Total net operating income									35,631	36,249	36,233
Reconciliation to net income:
Total Assets	313,783				323,792				313,783	323,792
Office Raleigh, NC [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									128,189	122,173	118,352
Total net operating income									95,926	88,402	86,053
Reconciliation to net income:
Total Assets	839,831				830,128				839,831	830,128
Office Richmond, VA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									48,079	49,428	45,729
Total net operating income									33,667	33,756	31,276
Reconciliation to net income:
Total Assets	240,976				246,546				240,976	246,546
Office Tampa, FL [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									99,520	86,431	102,404
Total net operating income									67,059	50,339	65,819
Reconciliation to net income:
Total Assets	556,951				521,620				556,951	521,620
Other Segment [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									32,609	74,717	75,429
Total net operating income									19,466	48,464	49,341
Reconciliation to net income:
Total Assets	322,670				415,376				322,670	415,376
Office Total Segment [Member]
Segment Reporting Information [Abstract]
Rental and other revenues									704,291	661,262	644,606
Total net operating income									485,609	439,004	$ 428,279
Reconciliation to net income:
Total Assets	$ 4,886,747				$ 4,722,868				$ 4,886,747	$ 4,722,868