Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 195,495	$ 181,043	$ 564,802	$ 556,996
Total Net Operating Income	134,928	124,151	391,820	382,783
Total Assets	5,754,072		5,754,072		$ 5,209,417
Reconciliation to net income:
Depreciation and amortization	(66,547)	(60,303)	(189,423)	(180,914)
Impairments of real estate assets	0	0	0	(1,778)
General and administrative expenses	(10,350)	(9,155)	(30,409)	(30,169)
Interest expense	(21,986)	(19,886)	(60,755)	(61,003)
Other income	424	(3,311)	1,068	(2,654)
Gains on disposition of property	38,572	10,012	80,371	163,397
Equity in earnings of unconsolidated affiliates	546	823	1,614	2,965
Net income	75,587	42,331	194,286	272,627
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	192,570	173,979	555,147	528,938
Total Net Operating Income	133,255	119,807	386,139	365,904
Total Assets	5,656,880		5,656,880		4,886,747
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,672	36,022	108,385	110,518
Total Net Operating Income	24,157	23,303	71,779	72,387
Total Assets	999,319		999,319		1,011,807
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,313	8,858	32,364	26,801
Total Net Operating Income	11,159	6,973	25,446	21,298
Total Assets	775,852		775,852		443,051
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,136	34,661	106,873	103,780
Total Net Operating Income	27,355	25,022	78,172	74,907
Total Assets	1,264,025		1,264,025		1,191,219
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,852	11,518	38,141	36,844
Total Net Operating Income	7,920	6,505	23,414	22,260
Total Assets	284,361		284,361		289,129
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,220	14,430	42,787	44,054
Total Net Operating Income	8,291	8,934	25,591	26,954
Total Assets	312,304		312,304		313,783
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	42,471	32,037	117,168	95,792
Total Net Operating Income	31,425	24,037	88,005	71,670
Total Assets	1,276,356		1,276,356		839,831
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,050	11,878	35,263	35,972
Total Net Operating Income	7,805	8,204	24,092	25,403
Total Assets	211,263		211,263		240,976
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	23,856	24,575	74,166	75,177
Total Net Operating Income	15,143	16,829	49,640	51,025
Total Assets	533,400		533,400		556,951
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	2,925	7,064	9,655	28,058
Total Net Operating Income	1,673	$ 4,344	5,681	$ 16,879
Total Assets	$ 97,192		$ 97,192		$ 322,670