Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 185,502	$ 183,153	$ 369,307	$ 375,953
Total Net Operating Income	129,276	128,034	256,892	258,632
Reconciliation to net income:
Depreciation and amortization	(61,949)	(59,461)	(122,876)	(120,611)
Impairments of real estate assets	0	(1,778)	0	(1,778)
General and administrative expenses	(10,107)	(10,084)	(20,059)	(21,014)
Interest expense	(19,001)	(19,840)	(38,769)	(41,117)
Other income	332	588	644	657
Gains on disposition of property	22,862	318	41,799	153,385
Equity in earnings of unconsolidated affiliates	431	1,179	1,068	2,142
Net income	61,844	38,956	118,699	230,296
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	182,204	175,543	362,577	354,959
Total Net Operating Income	127,209	123,424	252,884	246,097
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,738	36,537	71,713	74,496
Total Net Operating Income	23,622	23,901	47,622	49,084
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	8,917	9,010	18,051	17,943
Total Net Operating Income	7,035	7,123	14,287	14,325
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	35,578	34,622	70,737	69,119
Total Net Operating Income	25,824	25,132	50,817	49,885
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,730	12,304	25,289	25,326
Total Net Operating Income	7,782	7,750	15,494	15,755
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	13,949	14,676	28,567	29,624
Total Net Operating Income	8,411	9,350	17,300	18,020
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	37,655	31,202	74,697	63,755
Total Net Operating Income	28,444	23,997	56,580	47,633
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,751	11,834	23,213	24,094
Total Net Operating Income	8,234	8,508	16,287	17,199
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,886	25,358	50,310	50,602
Total Net Operating Income	17,857	17,663	34,497	34,196
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	3,298	7,610	6,730	20,994
Total Net Operating Income	$ 2,067	$ 4,610	$ 4,008	$ 12,535