Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 181,043	$ 187,475	$ 556,996	$ 543,908
Total Net Operating Income	124,151	123,340	382,783	358,664
Reconciliation to net income:
Depreciation and amortization	(60,303)	(60,850)	(180,914)	(189,514)
Impairments of real estate assets	0	(5,318)	(1,778)	(5,849)
General and administrative expenses	(9,155)	(11,717)	(30,169)	(33,658)
Interest expense	(19,886)	(20,527)	(61,003)	(59,622)
Other income/(loss)	(3,311)	174	(2,654)	(3,271)
Gains on disposition of property	10,012	3,515	163,397	10,218
Equity in earnings of unconsolidated affiliates	823	940	2,965	2,369
Net income	42,331	29,557	272,627	79,337
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	173,979	168,703	528,938	487,907
Total Net Operating Income	119,807	111,344	365,904	322,539
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,022	39,120	110,518	113,115
Total Net Operating Income	23,303	24,372	72,387	72,614
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,858	0	26,801	0
Total Net Operating Income	6,973	0	21,298	0
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,661	34,748	103,780	99,256
Total Net Operating Income	25,022	25,191	74,907	71,717
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,518	12,854	36,844	39,781
Total Net Operating Income	6,505	7,889	22,260	24,452
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,430	15,154	44,054	45,463
Total Net Operating Income	8,934	9,150	26,954	27,179
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	32,037	30,688	95,792	90,458
Total Net Operating Income	24,037	22,066	71,670	65,363
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,878	12,576	35,972	37,203
Total Net Operating Income	8,204	8,337	25,403	25,499
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	24,575	23,563	75,177	62,631
Total Net Operating Income	16,829	14,339	51,025	35,715
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	7,064	18,772	28,058	56,001
Total Net Operating Income	$ 4,344	$ 11,996	$ 16,879	$ 36,125