Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	$ 183,153	$ 184,070	$ 375,953	$ 356,433
Total Net Operating Income	128,034	123,512	258,632	235,324
Reconciliation to net income:
Depreciation and amortization	(59,461)	(59,460)	(120,611)	(128,664)
Impairments of real estate assets	(1,778)	(531)	(1,778)	(531)
General and administrative expenses	(10,084)	(9,560)	(21,014)	(21,941)
Interest expense	(19,840)	(20,356)	(41,117)	(39,095)
Other income/(loss)	588	321	657	(3,445)
Gains on disposition of property	318	6,703	153,385	6,703
Equity in earnings of unconsolidated affiliates	1,179	765	2,142	1,429
Net income	38,956	41,394	230,296	49,780
Total Office Segment [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	175,543	165,301	354,959	319,204
Total Net Operating Income	123,424	111,268	246,097	211,195
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	36,537	37,692	74,496	73,995
Total Net Operating Income	23,901	24,819	49,084	48,242
Office Charlotte, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	9,010	0	17,943	0
Total Net Operating Income	7,123	0	14,325	0
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	34,622	33,109	69,119	64,508
Total Net Operating Income	25,132	24,208	49,885	46,526
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	12,304	12,995	25,326	26,927
Total Net Operating Income	7,750	8,007	15,755	16,563
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	14,676	14,994	29,624	30,309
Total Net Operating Income	9,350	8,895	18,020	18,029
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	31,202	30,673	63,755	59,770
Total Net Operating Income	23,997	22,438	47,633	43,297
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	11,834	12,341	24,094	24,627
Total Net Operating Income	8,508	8,610	17,199	17,162
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	25,358	23,497	50,602	39,068
Total Net Operating Income	17,663	14,291	34,196	21,376
Other [Member]
Segment Reporting Information [Line Items]
Total Rental and Other Revenues	7,610	18,769	20,994	37,229
Total Net Operating Income	$ 4,610	$ 12,244	$ 12,535	$ 24,129