Segment Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Abstract]
Rental and other revenues	$ 768,007	$ 736,900	$ 735,979
Total net operating income	531,571	505,075	487,468
Reconciliation to net income:
Depreciation and amortization	(259,255)	(241,585)	(254,504)
Impairments of real estate assets	0	(1,778)	(5,849)
General and administrative expenses	(40,553)	(41,031)	(44,067)
Interest expense	(85,853)	(80,962)	(81,648)
Other income/(loss)	1,394	(1,707)	(2,510)
Gains on disposition of property	174,059	215,897	39,517
Equity in earnings of unconsolidated affiliates	1,947	4,005	3,276
Net income	323,310	357,914	141,683
Total Assets	5,695,138	5,209,417
Office Atlanta, GA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	143,612	146,704	151,279
Total net operating income	94,122	95,448	97,019
Reconciliation to net income:
Total Assets	947,877	1,011,807
Office Charlotte, NC [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	49,347	35,733	4,650
Total net operating income	38,464	28,431	3,791
Reconciliation to net income:
Total Assets	771,121	443,051
Office Nashville, TN [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	149,674	138,089	133,867
Total net operating income	110,039	99,901	97,386
Reconciliation to net income:
Total Assets	1,294,178	1,191,219
Office Orlando, FL [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	51,281	49,459	52,679
Total net operating income	31,301	29,546	32,062
Reconciliation to net income:
Total Assets	285,781	289,129
Office Pittsburgh, PA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	57,371	58,518	60,755
Total net operating income	34,248	35,631	36,249
Reconciliation to net income:
Total Assets	310,296	313,783
Office Raleigh, NC [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	162,115	128,189	122,173
Total net operating income	121,005	95,926	88,402
Reconciliation to net income:
Total Assets	1,269,200	839,831
Office Richmond, VA [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	45,941	48,079	49,428
Total net operating income	31,726	33,667	33,756
Reconciliation to net income:
Total Assets	202,488	240,976
Office Tampa, FL [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	97,954	99,520	86,431
Total net operating income	64,396	67,059	50,339
Reconciliation to net income:
Total Assets	514,303	556,951
Other Segment [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	10,712	32,609	74,717
Total net operating income	6,270	19,466	48,464
Reconciliation to net income:
Total Assets	99,894	322,670
Office Total Segment [Member]
Segment Reporting Information [Abstract]
Rental and other revenues	757,295	704,291	661,262
Total net operating income	525,301	485,609	$ 439,004
Reconciliation to net income:
Total Assets	$ 5,595,244	$ 4,886,747